Cleanup utility

[frisi]

if plone objects get deleted, we still have records for those in the mustread database.
currently these are simply logged with a warning (e.g. in what_to_read())

it might be a good thing to keep an audit log of all mustread actions over the time, no matter if the objects maked as read/not read have been deleted in the meantime.
but having a cleanup utility might be a good thing to have too.

cleanup procedure

• check for all mustreadrecors having path beginning with plonesite path
• group by uid
• api.content.get(UID=uid) if None, remove all records for this uid

i don't have a bloody need for this atm, just adding it as an enhancement idea