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
What would you recommend to solve the problem of showing 'Add To Leaderboard' button with `force_submission_to_leaderboard = True`?

Simply hiding it seems OK to me, for the sake of getting this merged sooner-than-later.