Beta version of a Gradebook.

Gradebook view renders student login names vs. assignment scores. Columns are all sortable asc/desc.

Some more things to do:
* Link the column heading name to the student progress view containing statistics for all students for the set corresponding to the column heading name.
* Link the row label name (login name) to the student progress view containing statistics for all sets for the student with login name corresponding to the row label name.

Questions:
* Should we make certain CRUD functions available on this page, like delete assignment or delete student?
 - for example highlighting a column and pressing a delete button removes the set for all users, or have an icon in the column heading that when pressed, deletes the set for all users.
 - similarly for deleting students