Create a `cache_key` function.

Create a function to simplify the declaration of cache keys.

Combine all the keys necessary for a student into a single view. The
`cache_key` method of `StudentData` will build a key using strings and
symbols, where the symbols represent cache keys generated by the
`membership_calculation` view.

The `membership_calculation` will be expanded to include any
calculations that can be performed by PostgreSQL for a particular
student for a particular course.

Closes #530.
Remove unused `student_cache_keys` view.

See #530.
Merge pull request #572 from bellotastudios/assignment-sums

[Reviewed] Create a `cache_key` function.