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
> Re: starting migration after submissions already in that phase. Is this a problem?

Yes, because the point of this feature is to make sure that the last successful submission of phase N becomes the first submission of phase (N+1) in order to generate a baseline result in phase (N+1).