Changed compare dropdowns to dropdowns with search input

## What does this MR do?

This changes the compare dropdowns from text inputs, that when clicked, open a dropdown of branches/tags to dropdowns that have a dropdown toggle and an isolated search input.

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

## Why was this MR needed?

This was needed to fix the poor UX highlighted in #22221, where opening the dropdown showed an initially filtered set of results because the dropdown toggle was the dropdown filter itself. The compare page is always loaded with `master` as each branch/tag selection, so when opening the dropdown, it would only show results matching `master`.

## Screenshots (if relevant)

![2016-09-27_18.28.10](/uploads/0ea1d91cb592c6e140ed62c336e77227/2016-09-27_18.28.10.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 [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] 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)

## What are the relevant issue numbers?

Closes #22221

See merge request !6550
Signed-off-by: Rémy Coutable <remy@rymai.me>