Cleanup of old user accounts

[mitar]

A regular (daily?) Celery task (with manage.py interface to call directly if Celery is not running) should be defined which would cleanup/purge old user lazy accounts (not linked to any 3d party authentication or without a password, is_anonymous is True).

Last login time should be consulted for this and last activity on the site.

It should be optional what should be done with possible content the user has created:

• delete it
• do not do anything with such lazy users
• reassign this content to some other user (like "deleted")