Fixed merge on success widget buttons on mobile

## What does this MR do?

Fixes 'merge on success' widget buttons on mobile as they were stacked with 100% width before when it is a button group with a small dropdown button.

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

Ney

## Why was this MR needed?

mobile UX

## What are the relevant issue numbers?

Closes #18610.

## Screenshots (if relevant)

![Screen_Shot_2016-06-17_at_03.09.36](/uploads/b662e1b7ad683cd89f1233a859caccb2/Screen_Shot_2016-06-17_at_03.09.36.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 !4726