Fixed notes action buttons on mobile

## What does this MR do?

Fixes the mobile screen size styling for notes action buttons. It ensures there is space between icons and forces parts of the notes header to stack

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

Stacking? 

## Why was this MR needed?

UX

## What are the relevant issue numbers?

Closes #18434.

## Screenshots (if relevant)

![Screen_Shot_2016-06-13_at_17.52.09](/uploads/18849fdfc4fcbf7b67fc66633bbac129/Screen_Shot_2016-06-13_at_17.52.09.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 !4631