Resolve "Inconsistency: Repo Title "down caret" cropped on smaller viewports"

## What does this MR do?

Fixes an issue where the dropdown chevron for a project list was not visible on smaller viewports.

This MR also changes the dropdown trigger into a `button` element for better accessibility (it can now be tabbed to like any link).

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

None that I can think of.

## Why was this MR needed?

Better accessibility and support for smaller viewports.

## Screenshots (if relevant)

Dropdown toggle now targetable by pressing tab to select.
![Screen_Shot_2016-08-31_at_9.12.15_PM](/uploads/8b2038a8ae259efd32da2e94a309ce88/Screen_Shot_2016-08-31_at_9.12.15_PM.png)

Chevron no longer hidden when project title length triggers `overflow: ellipsis`.
![Screen_Shot_2016-08-31_at_9.13.38_PM](/uploads/382e76c06e54bc307106a84ec21ab892/Screen_Shot_2016-08-31_at_9.13.38_PM.png)

## Does this MR meet the acceptance criteria?

- [x] CHANGELOG entry added
- Tests
  - [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)

## What are the relevant issue numbers?

Closes #21506

See merge request !6140