Users who have taken actions in the system are now never dropped
automcatically. For the moment the actual dropping logic is commented out
so consequences for production can be checked beforehand
This entailed adding a new routine in include/Cron.php that checks for old
users and purges their username and notes.
As part of this some other changes were necessary:
- Introduced a creation time for users, so it's possible to distinguish
between an old user with no loans and a new user with no loans.
- The user.name field in the database now allows null values.
