Advanced Competition Create

This pull request integrates a couple of changes:

1) Competition creation is delegated to a celery task to be done asynchronously.
2) Competitions can be created with complex leaderboards.

TODO:
- [ ] @cpoulain Ordering column groups in the leaderboard is not possible now, that I can find. I suspect it should be.
- [ ] With the new asynchronous model it would be good to find a notifications solution so that the backend application can notify the user of things running, failing, or completing.