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
it looks like this file was edited directly. it's actually a generated file, along with its minified version which the base template references. you'll want to edit `apps\web\static\js\Competition.js`, then run the generation tools per https://github.com/codalab/codalab/wiki/Dev_Getting-Started-on-Windows#install-the-javascript-closure-compiler-optional.