Follow-up from "Resolve "Add external_url for requirements controls""

The following discussion from !179256 (merged) should be addressed:

• @mwoolf started a discussion: (+3 comments)

  suggestion: Should we add a spec for a localhost, and other non-valid URL types?

In a .com? context we would want to not allow local addresses. For SM(self managed versions of GitLab) it should still be allowed since there could be other services on the same host used.