Delete account information for unused accounts

[root]

An important obligation under privacy legislation is to get rid of old data that is no longer required. This is especially true of personal data.

So we need to implement some sort of capability that removes all user accounts that have shown no activity for some period (e.g. 2 years).

1. This should be an automated process (ie. cron job)
2. The inactivity period should be configurable (in config.php)
3. Two different inactivity periods should be configured:
    3a. For non-disabled users
    3b. For disabled users
4. The user classes to which this applies should be configurable (by default, it should not apply to Admins)

[Adam P (adam+)]

See also https://bugs.launchpad.net/psiphon/+bug/457345

[Chris (poser)]

...of all _disabled_ accounts that have shown no activity...

[Chris (poser)]

Also, a user should have the option to fully delete his or her account, rather than just disabling it and waiting for this (as yet hypothetical) time-out.

[Rod (rod-psiphon)]

Operational database backup policy may need to be changed to meet policy requirements. I.e., destroy older backups after some point in time so old copies of deleted user information is not retained.