Grade entry

This consists of continued work on https://github.com/MarkUsProject/Markus/pull/1068, for https://github.com/MarkUsProject/Markus/issues/49

Quoting Mike:

> Creates a Grader tab in the Grade Entry view, allowing instructors to assign TAs to students on a many-to-many
> basis. Includes searching, sorting, CSV upload/download, and implements the new permissions in the Grade
> Entry table.
> 
> Fixes issue #49.