overrides: Improve enablement

The current model API allows us to tag an override in Koji without
resetting its expired_date to None.

Instead, we should always reset the expired_date every time we tag the
build.

This commit does just that.
overrides: Add a script to expire them

This could be run as a cron job, to regularly expire (e.g once a day)
old buildroot overrides, preventing them from accumulating.