Add CE code missing from EE

As discovered in https://gitlab.com/gitlab-org/gitlab-ee/issues/715#note_16838348, it seems that some CE code is missing from EE.

It seems to happen for 33 files right now (but some of them are just files that are not yet in EE but that will be added on the next CE->EE merge).

We should add the missing code. Basically after resolving this, we should (ideally) not have any 0 N (CE code removed in EE) kind of diff here: https://gitlab.com/snippets/28629

• Use the same Rubocop config fro CE and EE: gitlab-org/gitlab-ee!835
• Backport Group API code that was added in EE only: gitlab-org/gitlab-ce!7205
• Remove dead code and align with some JS code from CE: gitlab-org/gitlab-ee!836