Added 100% width to open dropdown menus on mobile

## What does this MR do?

Sets dropdown-menus to be 100% width on small screens.

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

Do we want this on all dropdown menus or only dropdown menus activated by full-width buttons?
/cc @dzaporozhets 

## Why was this MR needed?

Mobile UX.

## What are the relevant issue numbers?

Closes #18858.

## Screenshots (if relevant)

![Screen_Shot_2016-06-22_at_00.14.09](/uploads/bd61721896bfac8af951374c2547acd6/Screen_Shot_2016-06-22_at_00.14.09.png)

## 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
- [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 !4844