Improvements to @Xyene's contest models.
Improvements to @Xyene's contest models.

1. Allowed contests, contest submissions, and contest history to be blank
2. Allowed contests to have descriptions.
3. Used a fancy TimedeltaField to do better time handling.
4. @property magic to automagically create a ContestProfile for user
5. Linked ContestParticipation to users

monouno/site,monouno/site,Phoenix1369/site,Phoenix1369/site,Phoenix1369/site,Minkov/site,Phoenix1369/site,DMOJ/site,monouno/site,monouno/site,DMOJ/site,Minkov/site,Minkov/site,DMOJ/site,monouno/site,DMOJ/site,Minkov/site