Rewrite `if:` argument in before_action and alike when `only:` is also used

This is first discovered in https://gitlab.com/gitlab-org/gitlab-ce/issues/54739#note_122609857 that if both if: and only: are used in a before_action or after_action or alike, if: is completely ignored.

I hit into the same question at https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8903/diffs#note_126164835 where I am trying to figure out what it means when we pass an array to if:, and I don't seem to find if it'll mean anything, even looking at the source.

I doubt if it's just ignored and doesn't do anything.

To avoid confusion, we should remove all arguments which don't do anything. The original code was also pretty ancient and I didn't bother to keep digging the history.

Updated: We decided to rewrite it to better reflect the intention.