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 ⚔ 

## 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