Send notification emails when users are newly mentioned in issue or MR edits

## What does this MR do?

Introduces "new mention in issue" and "new mention in MR" email notifications.  Editing a Mentionable title or description and adding a mention to a user who was not previously mentioned will now send them a notification email, following usual permissions for doing so.

## Why was this MR needed?

Issues & MRs may be edited to include mentions to new people. We don't currently send out email notifications of these edits to anyone, although they do create TODOs. This brings email notifications into parity with TODOs.

## What are the relevant issue numbers?

Closes #2451

See merge request !5800