Sort user list by admin first, then by username

Right now, users show in in the right panel almost at random -- I believe the correct order should be admins at the top, followed by non-admins, and then within those groups you sort by username. See below.

Before:

![screen shot 2014-12-08 at 2 15 55 pm](https://cloud.githubusercontent.com/assets/463175/5345343/18a55016-7ee5-11e4-8606-8be75d39ce97.png)

After:

![screen shot 2014-12-08 at 2 14 59 pm](https://cloud.githubusercontent.com/assets/463175/5345350/1dcb8948-7ee5-11e4-8e9a-fd5016f5bc02.png)