Auto-migration timing

[Didayolo]

The task performing phase migration is scheduled every 300 seconds:

codabench/src/settings/base.py

Line 225 in fda56ab

CELERY_BEAT_SCHEDULE = {

That can be confusing

• If feasible, we should trigger the phase migration check at the moment a phase starts. This way we do not wait, and do not run the tests every 300 seconds on the site worker.
• Otherwise, let's add a warning such as: "The migration may start up to x seconds after the beginning of the phase" near the option.