Adding functionality for Remove Submission from grading pool.

This would maintain the history for deleted submissions via new database table.
TNL-900
Add rule for .staff-info__collapse__title here to add the 10px margin-left being added to the other collapsible header for the parent "COURSE STAFF INFORMATION SECTION"  (Here's an example of what I mean for the parent header area: https://www.dropbox.com/s/nkh3lyzutmtceaj/ora-margin-left.png?dl=0)