Factor zen mode.
Created by: cirosantilli
Almost the same code was on 3 different places.
Minor UI change: both new issue and merge request description textarea are now the same size. They were different sizes, so I picked an intermediate new size. The change is almost unnoticeable.
The pages that could be affected are: new / edit issue and merge request, diff notes and comments.
This continues the move for form DRYness as in https://github.com/gitlabhq/gitlabhq/pull/7678
cc. @Razer6 if you get the time please check that I didn't break the feature =) A good indicator of non-DRYness are strings shown on the UI: when I saw that there were two occurrences of "Edit in fullscreen", I immediately git grep -C2 'Edit in
and found this opportunity for DRYing.