Backport common class name for the tab content

Backport more changes

 - Split layout from the styles of the row
 - Move command `.append-` `.prepend-` styles to bottom of source order
    - Actually applies styles over others in more situations which is
      probably what is expected when you add it to an element
    - Also allows us to get rid of some styles that were trying to get
      around this fact

See https://i.imgur.com/IE50PK6.png
Which is used in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15064

Design from https://gitlab.com/gitlab-org/gitlab-ce/issues/38464

CSS improvements after review