Consider a rubocop to enforce fully-qualified constants in ee/ directory

As https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5644 shows, sometimes Ruby's name lookup does unexpected things that can bite us.

We should consider a rubocop that will complain for any non-fully-qualified constants in the ee/ directory.

For example, if working on EE::Epics, the Issuable constant should be written as ::Issuable. The EE::Issuable constant as EE::Issuable, etc.