CLC-3399 - Need ability for TA/GSIs to be able to view Roster photos

https://jira.ets.berkeley.edu/jira/browse/CLC-3399

* Added methods to Canvas::CoursePolicy to determine if user is a course teacher or assistant
* Added Canvas::CoursePolicy#can_view_course_roster_photos? for Pundit authorization
* Added Berkeley::Course and Berkeley::CoursePolicy for use with CalCentral Roster photo authorizations
* Refactored CanvasRostersController, CampusRostersController, and the RostersController, to use Pundit authorizations
* Refactored Canvas::CanvasRosters, Rosters::Campus, and Rosters::Common to serve roster feed and photos without authorization code