Use NotificationSetting::EMAIL_EVENTS for params

Use gitlab-workhorse 0.8.1

Adds Sentry support in gitlab-workhorse.

See merge request !6283

This adds (basic) Sentry support.

Minor update on CI docs examples

cc @ayufan @markpundsack 

Commit message: 

> Triggered by gitlab-org/gitlab-ci-yml#1 I've investigated how this is
organized so far and did a minor clean up. In my opinion we should just
keep a list of up to date blog posts in the docs and examples should be
included in gitlab-org/gitlab-ci-yml. This reduces the number of
projects and docs pages one has to look for information.

See merge request !5806

Security and safety improvements for gitlab-workhorse integration

Companion to https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/60

- Use a custom content type when sending data to gitlab-workhorse
- Verify (using JWT and a shared secret on disk) that internal API requests came from gitlab-workhorse

This will allow us to build features in gitlab-workhorse that require
more trust, and protect us against programming mistakes in the future.

This is designed so that no action is required for installations from
source. For omnibus-gitlab we need to add code that manages the shared
secret.

See merge request !5907

Triggered by gitlab-org/gitlab-ci-yml#1 I've investigated how this is
organized so far and did a minor clean up. In my opinion we should just
keep a list of up to date blog posts in the docs and examples should be
included in gitlab-org/gitlab-ci-yml. This reduces the number of
projects and docs pages one has to look for information.

[ci skip]

Allow compare merge request versions

## What does this MR do?

Add new functionality to the merge request page. It allows you easily compare merge request versions in one click.  

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

## Why was this MR needed?

To improve code review experience. 

## Screenshots (if relevant)

See discussion

## Does this MR meet the acceptance criteria?

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

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/13570

See merge request !6127

Fix: Switch sample values for $CI_SERVER_REVISION and $CI_SERVER_VERSION.

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html.

## What does this MR do?

There is a sample in this document showing the possible values of the predefined CI variables. Those of **CI_SERVER_REVISION** and **CI_SERVER_VERSION** were mixed up. I fixed that.

See merge request !6243

API for CI Lint

## What does this MR do?
Add API for CI Lint. Can check if `.gitlab-ci.yml` content is valid.

## What are the relevant issue numbers?
Closes #15571 

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] API support added
- [x] Documentation
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing

cc @ubudzisz @grzesiek @yorickpeterse

See merge request !5953