A rework of the purge processes is needed

With the new retention, space and migration features, a few new cases have to be handled by the purge system:

New mail model, for the projects in shortterm storage (4 months), that indicate 1 month in advance that the data are going to be deleted. No extention is allowed in this case.
A new CRON that sends the purge mail one month before the data deletion and opens a purge request (Admin > Mail obsolete projects).
A new CRON (every week) to send to the ng6 administrators the list of the last purge mails sent.