Small UI improvements to merge request page

Make gray block under tabs identical height. Also same height have blocks on pages like `Groups` etc. So this improvement is targeted for consistency in UI too
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fixes internal issues 

* https://dev.gitlab.org/gitlab/gitlabhq/issues/2591
* https://dev.gitlab.org/gitlab/gitlabhq/issues/2592

cc @DouweM 


See merge request !1759