Fix phase UI

Resolves #585 add phase description as well.

## Tasks

- [x] Verify `phase.description` works in `competition.yaml`
- [x] Fix admin view phase list (changed to dropdown)
- [x] Fix phase wrap around (if no `previous-phase`, shows the `last-phase`)
- [x] Show `phase.label` instead of `phasenumber`
- [x] Verify `auto_migrate` still works