Move to project dropdown with infinite scroll for better performance

See merge request !5828

Use just SQL to check is a user can admin_issue on a project

Tradeoff

- we duplicate how we check admin_issue in a SQL relation in the Ability class

Fix error message for existing labels in right sidebar.

## What does this MR do?
Fixes flash notification in right sidebar, if we want to create an existing label.

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

## Why was this MR needed?
Because it was broken.

## What are the relevant issue numbers?
Fixes #20117 

## Screenshots (if relevant)

#### Before
![https://gitlab.com/gitlab-org/gitlab-ce/uploads/c1fa7e98d1a3f0daaf8197809bbb26d0/Screen_Shot_2016-07-22_at_11.33.35.png](https://gitlab.com/gitlab-org/gitlab-ce/uploads/c1fa7e98d1a3f0daaf8197809bbb26d0/Screen_Shot_2016-07-22_at_11.33.35.png)

#### After
![Screen_Shot_2016-07-28_at_13.38.03](/uploads/1dca73bf4c1b14c0044952a5032d9325/Screen_Shot_2016-07-28_at_13.38.03.png)
![Screen_Shot_2016-07-28_at_13.41.18](/uploads/9b814f7f03d0314050659e1ac6ace290/Screen_Shot_2016-07-28_at_13.41.18.png)

## Does this MR meet the acceptance criteria?

- [x] [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 !5546

Don't show comment button in gutter of diffs on MR discussion tab

When browsing to an MR, switching to the Changes tab, and switching back to the Discussions tab, the gutter of the diffs on that page would show the comment button on-hover, while it shouldn't, since comments on new lines can only be created from the Changes tab.

![Screen_Shot_2016-07-25_at_22.01.11](/uploads/6b044e1a3ecb34ddaa0af432833473aa/Screen_Shot_2016-07-25_at_22.01.11.png)

`8-10-stable` counterpart at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5493

See merge request !5492

Updated Akismet documentation

Updated Akismet documentation

See merge request !5821

[ci skip]

Add hover state to todos

## What does this MR do?

- Add a hover state to todos like for repository files.
- Fix button alignment in todos
- Add link to avatar in todos

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

The `border-collapse` part—is there any better way?

## Why was this MR needed?

Make hover states consistent.

## What are the relevant issue numbers?

fixes #19833 

## Screenshots

### Before

![before](/uploads/cd2e7b234bf08fac48aebb60cdfa26a9/before.png)

### After

![todos-list](/uploads/592457050cb3ff2c43dfcce78418c50b/todos-list.png)

![todos-list-last-hover](/uploads/532714333bc74d1656a5c767012b4cdd/todos-list-last-hover.png)

![todos-mobile](/uploads/d542b04ddebc9d4354d57151dd04c0d2/todos-mobile.png)

See merge request !5361