Merge branch '21285-new-merge-request-source-and-target-branch-fields-filters-both-source-and-target-list' into 'master'

Fixed issue where filtering one dropdown will filter another on the page

## What does this MR do?

Moved `.option-hidden` to exist only in `SELECTABLE_CLASSES` as `NON_SELECTABLE_CLASSES` is used to re-show previously hidden elements.

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

## Why was this MR needed?

## What are the relevant issue numbers?

Closes #21285.

## Screenshots (if relevant)

![2016-08-24_15.59.16](/uploads/8282bd6507d10de61097956a57bd63cc/2016-08-24_15.59.16.gif)

## 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
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] 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)
Closes #21285

See merge request !5985