Factor zen mode.

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.

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.