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: documentation
**I will get on that**

Re: setup scheduler for test
**So, add a scheduler event for codalabtest.org, qaintdev, etc?**

Re: frequency
**Not sure, I arbitrarily chose 3 hours to give plenty of time for a phase migration to finish**

Re: starting migration after submissions already in that phase
**Is this a problem? I don't think this will happen very often and if it does, what is the downside? You may have duplicate submissions?**

Good observations, thanks for being so thorough!