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 UpdateServices 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 (closed)