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 could start by only keeping competitions which have a phase where `auto_migration` is True. You might be able to trim the list further by using the `last_phase_migration` field.