Feature/at mention email

Sending email on @mentions, per https://www.pivotaltracker.com/story/show/78869546
For this test, it'd be great if we could verify that the two jobs in the queue are indeed for author and ... and who?

As an example, I read this and am unclear what the requirements are. I see 4 `author.username`s, one `commenter.username` and one `someOtherHandle`. Which two are expected to be enqueued? The author mentions should be de-duped into one, we don't send an email to the person making the comment (the commenter), and no user with the username `someOtherHandle` exists (presumably).