Give project selection dropdowns responsive width, long names wrap.

## What does this MR do?

Makes two dropdown menus (branch name selector & project selector) wider on larger screens, and allows their contents to wrap.

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

Sanity check. 

## Why was this MR needed?

Considered a regression, as users who relied on long branch names could no longer see them fully. 

## Screenshots (if relevant)

Before: 
![Screen_Shot_2016-08-30_at_2.50.43_PM](/uploads/e0252cb02c5f34f3c777d2d67028042f/Screen_Shot_2016-08-30_at_2.50.43_PM.png)

After:
![Screen_Shot_2016-08-30_at_2.49.35_PM](/uploads/b504e0f3e139c85699bc5aafb58199f3/Screen_Shot_2016-08-30_at_2.49.35_PM.png)

After (updated after fixes, including project selector): 

![Screen_Shot_2016-09-02_at_1.46.53_PM](/uploads/b480fc8193f713ce43518cb83886fd52/Screen_Shot_2016-09-02_at_1.46.53_PM.png)

![Screen_Shot_2016-09-02_at_1.50.26_PM](/uploads/f0d31702668e83e6c0c24782cc43fdbd/Screen_Shot_2016-09-02_at_1.50.26_PM.png)
## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- Tests
  - [x] Added for this feature/bug
  - [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 #15633

See merge request !6105