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
I am testing this more and more but it seems like the behavior is worse when I try to make it "smarter" by only finishing when evaluations are completed. I just had it fail because a submission never reached its Finished state, but it wasn't empty, and there were no more submissions to trigger the check... maybe if we had submission re-trying we'd at least always get to the "finished" state.

I think I'm going to revert back to how it was... will test a bit more...