Issue #689 - User preferences, Assignment/Submissions

User preferences in the Assignment->Submissions page.

- The number of entries per page is now saved for a specific user/assignment
- The sort order (sorting column name and order) is now saved for a specific user/assignment
- The user can now choose which columns to show/hide in the Submissions table via a pop-up triggered by clicking the pencil in the table's header.
- Every setting is stored in an individual cookie.

A "RJS" script is called to show/hide the table columns when the table is initially shown and when the pop-up's form is saved.

Related blog post: http://blog.markusproject.org/?p=4532