Add integrity constraints to database models

[TimJentzsch]

We have a couple of constraints that should always hold.
I'm not sure if it's possible to encode these in Django, but if it is, it would help to keep our data clean and potentially speed up some queries.

Note that we might need to fix some faulty data if we integrate this.

Some constraints that I can think of at the top of my head:

Submission:

• If claimed_by is not null, then claim_time is not null
• If completed_by is not null, then complete_time is not null
• If completed_by is not null, then claimed_by is not null
• create_time <= claim_time <= complete_time
• If removed_from_queue is true, then approved is false
• If approved is true then removed_from_queue is false