Content libraries permissions UI (WIP)

This adds a new settings page in studio's library view that can be used to assign roles to users of a content library.

Each user can be in one of three roles:
* __Admin__: Can edit content and assign roles to users
* __Staff__: Can edit content but not users
* __User__: Can view content (and use it in their own courses) but cannot edit anything.

![screen shot 2014-11-12 at 11 15 29 pm](https://cloud.githubusercontent.com/assets/945577/5034115/b2ff27f0-6b20-11e4-90ae-7dc0fcbd0796.png)