Consider this command:

    bundle exec rails r "include GitlabMarkdownHelper
    puts markdown('<span>this is a span</span>', pipeline: :description)
    puts markdown('<span>this is a span</span>')"

And the same in the opposite order:

    bundle exec rails r "include GitlabMarkdownHelper
    puts markdown('<span>this is a span</span>')
    puts markdown('<span>this is a span</span>', pipeline: :description)"

Before this change, they would both output:

    <p><span>this is a span</span></p>
    <p>this is a span</p>

That's because `span` is added to the list of whitelisted elements in
the `SanitizationFilter`, but this method tries not to make the same
changes multiple times. Unfortunately,
`HTML::Pipeline::SanitizationFilter::LIMITED`, which is used by the
`DescriptionPipeline`, uses the same Ruby objects for all of its hash
values _except_ `:elements`.

That means that whichever of `DescriptionPipeline` and `GfmPipeline` is
called first would have `span` in its whitelisted elements, and the
second wouldn't.

Fix this by creating an entirely separate hash, before either pipeline
is invoked.

Add Issue/PR Templates to deter issues/contributions on the GitHub mirror of the project

## What does this MR do?
Adds GitHub-specific `ISSUE_TEMPLATE.md` and `PULL_REQUEST_TEMPLATE.md` files in a `.github` directory. To prevent new issues/PRs, I figured it'd be good to direct users/contributors to open issues/contribute code in the "correct" project.

## Are there points in the code the reviewer needs to double check?
Wording/phrasing, mostly.

## Why was this MR needed?
The GitHub issue tracker is being closed, and PRs on GitHub haven't been accepted for a while now. This was discussed briefly during the GitLab Strategy Session at the Austin Summit.

cc: @dzaporozhets @rymai  @MrChrisW @dblessing @virtuacreative @amara  

See merge request !4324

Enable RuboCop for migrations

## What does this MR do?

Enable RuboCop for all files inside `db/migrate`, then add magic comments to all existing files, so that this only affects new migrations.

## Are there points in the code the reviewer needs to double check?

This entire change is a config change and a bunch of comments.

## Why was this MR needed?

```
Yorick Peterse [11:55 AM]  
I don't think we have any use case for nested def, might as well blacklist it

Sean McGivern [11:57 AM]  
http://www.rubydoc.info/gems/rubocop/RuboCop/Cop/Lint/NestedMethodDefinition

Sean McGivern [11:57 AM]  
hmm, it's already enabled

Sean McGivern [11:57 AM]  
... because we exclude `db/` from rubocop :slightly_smiling_face:

Douwe Maan [11:57 AM]  
@smcgivern: heh

Sean McGivern [11:59 AM]  
I guess that's because we don't want to change the old migrations? I wonder if it's worth enabling it and adding magic comments to all the previous ones to ignore rubocop

Douwe Maan [11:59 AM]  
@smcgivern: agreed
```

## What are the relevant issue numbers?

None.

## Screenshots (if relevant)

None, but if I remove the magic comment from the migration `20160416182152_convert_award_note_to_emoji_award.rb` I get:
```
$ be rubocop
Inspecting 1959 files
..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................W....................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Offenses:

db/migrate/20160416182152_convert_award_note_to_emoji_award.rb:3:5: W: Lint/NestedMethodDefinition: Method definitions must not be nested. Use lambda instead.
    def up ...
    ^^^^^^

1959 files inspected, 1 offense detected
```

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- [ ] Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4559

Wrap all rate limiting logic inside GitHub API client

## What does this MR do?

Move the actual rate limiting logic to GitHub API to clean the code inside the GitHub importer, and avoid code duplication.

## Are there points in the code the reviewer needs to double check?

No there aren't.

## Why was this MR needed?

Avoid code duplication to handle API rate limit in every call to the GitHub API.

## What are the relevant issue numbers?

There are none.

## Screenshots (if relevant)

Not relevant.

See merge request !4552

Fix failing `EmailOnPush` spec.

Closes #18447 

- This should fix CI on master

/cc @smcgivern @ayufan @stanhu @pacoguzman 

See merge request !4582

Award emoji fixes

Fixes #18357 #18325 and #18424

See merge request !4550

yes -> true
no -> false
. frequent -> .frequent-emojis

[ci skip]

Properly quote table name in Rake task for MySQL and PostgreSQL compatibility

!4318 broke the gitlab:db:drop_tables functionality for PostgreSQL.
    
Closes #15259

See merge request !4577