Add account expiry system

[itsthejoker]

Default to 3 years, but allow configuration. Emails should automatically be queued and sent when expiration is approaching. When an account is expired, it should lock.

[itsthejoker]

...and by "lock", I mean the account should be marked as inactive using the flag that's already there. Error messages should be added to staff checkout, self checkout, and the login screen to denote an inactive account and that the account holder needs to have their account reactivated.