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
Several comments here:
- Scheduler setup needs to be documented in deployment section. @willum070 can help.
- There needs to be one scheduler per deployment (test vs. prod). Scheduler setup in CodaLab Azure account.
- What's the right frequency for the scheduler to do the check? Every minute?
- What happens if the migration starts after the phase has become active? There is the possibility that someone would submit in the interim unless we prevent it. Should the phase become active only after the migration has started, or do we prevent submission until migration has happened?