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
We need to verify this assumption. Also, if we grab the last one, we probably need to check the state of the submission (e.g. should the submission be in the Finished state);note that it could significantly complicate the logic.