Fix alignment of icon buttons

## What does this MR do?

Fix misalignment of buttons with only an icon which was introduced by !5451.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/5451#note_14066627

## Screenshots

### Before

![with-text](/uploads/aa7f7a98e2015070689e762e0cf99844/witht-text.png)

![without-text](/uploads/72db7c54e3961fd0df34b0ea79455fd0/without-text.png)

### After

![with-text](/uploads/3f19e55773083bcb22f3909f6dc0d147/witht-text.png)

![without-text](/uploads/b6647d4d71eb5cd9d1712908d2b93665/without-text.png)


See merge request !5887