Fix mentions not being created upon issue/merge request update

New cross-references weren't being added when they were made in an issue or merge request update.

This happened because the relevant `UpdateService`s were making the `notice_added_references` call
after the model had already been updated and saved, so the `changes` attribute was empty and no
cross-references were made at all.

This fixes the bug and adds a bit of testing and a bit of refactoring.

Closes #1773

See merge request !974