Fix an issue with methods not accessible in some controllers

Backport of !295 (merged) (actually I could have opened it against master at that time, I was probably confused by https://gitlab.com/gitlab-org/gitlab-ce/issues/14503#note_4386197).

I also cleaned the routing/controllers for audit_events and added RSpec features specs.

@DouweM I addressed your remarks from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/295. ;)