Closes #19573

Revert "Optimistic locking for Issue and Merge Requests"

The migration to add `lock_version` in !5146 to every issue and merge request takes too long on GitLab.com since it has to add a default value of 0 to every row. Unfortunately, Rails 4.2 doesn't work properly if the value is left as `nil`; anything using optimistic locking cannot be edited. 

This bug was fixed in Rails 5.0 via this commit: https://github.com/rails/rails/commit/13772bfa49325a8dc26410d2dcb555665a320f92. I suggest we revert this change for now, and when we upgrade to Rails 5.0 we can reintroduce this feature.

See merge request !5245

Update rubocop to 0.41.2

Adds a couple of cops to the TODO files, but I wanted [a specific cop](https://github.com/bbatsov/rubocop/pull/3135/files) which just had about 65 offences, so Ill do that later.

See merge request !4999

Disable PostgreSQL statement timeout during migrations

Long-running migrations may take more than the timeout allowed by the database. Disable the session's statement timeout to ensure migrations don't get killed prematurely.

See merge request !5263

Fix issues last update timestamp not preserved after importing a project

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19689 and https://gitlab.com/gitlab-org/gitlab-ce/issues/19792 (similar, but not the same)

Now `updated_at` should be the time from the original project.

See merge request !5253

Reduce padding on collapsed diff message

## What does this MR do?

Reduce the padding on the collapsed diff message so it takes up less space.

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

## Why was this MR needed?

Collapsed diffs took up too much space before.

## Screenshots (if relevant)

Before:

![image](/uploads/62c6fe19439bc3bc91bde83df72e7004/image.png)

After:

![image](/uploads/ec89230065c37fc4022d949d1233a90d/image.png)

## 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~~
  - [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)

See merge request !5256

Changed collapsed assignee tooltip to users name

## What does this MR do?

- Changes the collapsed assignee tooltip the users name rather than the username
- Updates the tooltip after updating the assignee

## What are the relevant issue numbers?

Closes #19280, #19281

## Screenshots (if relevant)

![Screen_Shot_2016-06-28_at_17.55.52](/uploads/e6ccc1703f59966c978e731a841aabdc/Screen_Shot_2016-06-28_at_17.55.52.png)

See merge request !4976

navbar_icon was renamed to custom_icon

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4909 it's still using `navbar_icon` but now it's renamed to `custom_icon` in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5156

/cc @iamphill @smcgivern @annabeldunstone 

See merge request !5265

Address UI feedback for admin area

## What does this MR do?
Addresses @dzaporozhets's feedback for !4424

## Why was this MR needed?
!4424 was merged with some design issues which affected other areas of the app and broke consistency. This MR fixes that.

## What are the relevant issue numbers?
#3946 

## 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
  - [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)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5190

Update permissons links to new page

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5210 updated the location in the docs, but we link to this page a bunch in the app too.

See merge request !5247