For a user with the mention notification level set, the type of their
corresponding NotificationRecipient must be :mention for them to receive an
email.

We set this correctly on notes, but we weren't adding it on new issues or MRs -
perhaps because these users are also participants. But the type of the
NotificationRecipient in that case would be :participant, not mention, so we
have to add the mentioned users manually when creating an issue or MR.

When editing an issue or MR, and there are newly-mentioned users to email, we
still use the :new_issue and :new_merge_request actions, so this works for that
case as well.