Send e-mail to STUDIO_REQUEST_EMAIL when user requests course creator privileges

This feature story is STUD-585.

In addition, I fixed the following bugs (with no associated JIRA records):

1. E-mail to user was not being sent on the transition from PENDING -> DENIED. It was only being sent when moving into or out of the GRANTED state (in all my demos, I always granted access first and then denied it...).

2. Change the admin view to display and sort on user.username instead of user. The sorting on user appears random (though I'm sure it isn't... we just don't understand the algorithm).