Fix up responsive mode in diff tabs for both Diff File Header and Compare Versions Header
We have done some work to have less clutter on the Compare Versions header (see https://gitlab.com/gitlab-org/gitlab-ce/issues/55491), but after adding the diff statistics, it is getting crowded on medium screens. On top of that, we're hiding all the information on the right on the smaller screens.
If something is important on a large screen, it is important on a small screen as well. We need to come up with a responsive solution, that elegantly displays all of the information we are showing on large screens.
Screen_Recording_2019-02-04_at_2.37.37_PM
Possibly related: https://gitlab.com/gitlab-org/gitlab-ce/issues/57250
/cc @andr3