Fix branches page dropdown sort alignment

## What does this MR do?
Makes the dropdown sort top aligned with the other elements beside it

## Are there points in the code the reviewer needs to double check?
Shouldn't be

## Why was this MR needed?
Improves the UI

## What are the relevant issue numbers?
Closes #20837 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-11_at_10.08.56_AM](/uploads/d446c65276d856f6257921c0e662d110/Screen_Shot_2016-08-11_at_10.08.56_AM.png)
![Screen_Shot_2016-08-11_at_10.08.55_AM__2_](/uploads/015f0ac263281275405b127cd19718b4/Screen_Shot_2016-08-11_at_10.08.55_AM__2_.png)

After:
![Screen_Shot_2016-08-11_at_10.15.35_AM](/uploads/73a231d4e4d474b3cfbc6944ddd4f0e9/Screen_Shot_2016-08-11_at_10.15.35_AM.png)
![Screen_Shot_2016-08-11_at_10.08.36_AM__2_](/uploads/c5cda7911258766c02554f68ebd4bf82/Screen_Shot_2016-08-11_at_10.08.36_AM__2_.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/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)

See merge request !5777