Factor new issue form, edit and new merge request forms.
Created by: cirosantilli
There is a lot of code duplication in those views.
https://github.com/gitlabhq/gitlabhq/pull/7678 starts to factor out the new issue and edit MR, but there is still some parts missing.
The New merge request form has diverged even more, and is the one that looks better now, so we should make the others look like it.