Invite reviewers by email
We need the ability to enter an email when choosing a reviewer, beyond choosing an existing user.
The system will need to send out an email with a link to the app, so that the reviewer can sign up.
We'll also need to figure out a way to store that this email has been associated with that particular article, so that when they do sign up, they're already invited to review.
This obviously dependent on #26 (closed), so that emails go out in the first place.