Added shortcut to focus filters

## What does this MR do?

Adds shortcut `f` to focus filter inputs.

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

Is this a valid implementation?

Please check [this comment below](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4570#note_12359978)!

## Why was this MR needed?

Keyboard ninja UX :crossed_swords: 

## What are the relevant issue numbers?

Closes #18120.

## Screenshots (if relevant)

![Screen_Shot_2016-06-09_at_18.34.56](/uploads/d4cfb6ec340c118f5227be3f16ffbed1/Screen_Shot_2016-06-09_at_18.34.56.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
  - [ ] 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 !4763

Updated CHANGELOG

Added docs for filter shortcut

Moved CHANGELOG entry

Added phils fixes

Fixed search form submitting with empty input

Added review fixes

Readability and selector perf fix

Removed from window and used static method

Updated shortcuts.png for docs

Updated CHANGELOG

Moved CHANGELOG entry

updated shortcut docs

- Need the `mobile?` detection (that the new version provides) for the
  U2F registration/ authentication flow