Add a way to get the latest status of submissions.

Use case: Queue is used to sign off students. Student signs up with a "submission" request type, and is handled by a TA. Last request of every assignment is selected and its status is logged in a CSV dump.

Things to note:

• The value for a student-assignment combination can by any value in the Requests.Status enum, or the special "NO_REQUEST" value if there has never been a request in the queue for this student for this assignment.
• Dump only lists the status of the last request, so if a student is accepted but later signs up again, the accepted status is dropped in favor of the new status
• If the latest request is changed after the fact, the dump will show the latest, updated status.
• Users who never signed in to the queue in the first place will not be part of the dump, so if these exist in BS, rules should be set properly for them to get a failing grade.

Fixes #111 (closed)