Resolve "Email notifications not sent as expected"

What does this MR do?

Sends emails to users with the mention level set on new issues and MRs correctly.

Are there points in the code the reviewer needs to double check?

I don't think so, but you might want to read NotificationRecipient to understand it.

Why was this MR needed?

If you had the mention level set as your notification level, and someone mentioned you in an issue or an MR description, you wouldn't get an email, which seems counterintuitive.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #38862 (closed).