Cache views.

Created two views that encapsulate the cache keys for students and
courses. May amalgamate them into a single view to save a trip to the
server, or have the student view include the courses view, in the
future. For now, it is better than having SQL in controllers.

See #528.
Add submissions cache key to syllabus.

Added the submissiosn cache key to the syllabus parital cache key.

Closes #528.
Merge pull request #529 from bellotastudios/submissions-cache

Submissions cache