Modifying assignments removes all the old ones

When you modify an assignment, the old assignments are removed and new assignment db objects are created. This leads to:

• Labs have their assignments reset, so you need to set those again
• For filtering, this isn't great because you cannot filter on assignmentID anymore since old requests still have the old FK to the assignment (#45 (closed) / #46 (closed)) (but you can of course filter on the name, as is done now).

Ideally, I would propose to not remove all assignments on modification, but just keep them and update them with the new values, if that would be acceptable.