Migrating submissions from old phase to new phase

Ready for functional review/QA/staging!

- [x] Use leaderboard entry as submission to move, if no leaderboard entry grab latest
- [x] `auto_migration` phase flag
- [x] `http://domain.com//competitions/check_phase_migrations` url, pinged every 3 hours
I couldn't think of a decent way to tell when all of the submissions were done at the time.

I thought about it for a bit and in my latest [commit](https://github.com/Tivix/codalab/commit/b89865428f03883cc7b1d5a6e6d0a9141ac28478) I think I've made this quite a bit more robust. If previous phase has no submissions -> marked completed. When all submissions are in `FINISHED` state -> migration marked completed.