Implemented Course Section create and delete interface.

NEW
* Implemented the `create` and `delete` functionality for the
  Course Section administrative interface.
* Created the CourseSection controller.

CHANGES
* Created a new route filter to deal with setting an appropriate
  school session and keeping track of it.
* Updated the database seed so that there is a default school
  session.
* Changed the School Session controller so that the default
  session cannot be deleted.