Fix rubocop offenses

Hiya!

This MR updates RuboCop (in sync with gitlab-org/gitlab) and fixes all the RuboCop offenses.

I understand that this MR is quite huge, I wouldn't mind splitting this into multiple sets.
Let me know if you want me to do so.

CC: @rpaik^

Signed-off-by: Utkarsh Gupta <utkarsh@debian.org>