Fix #232, Implement 'Paranoid' gem

* Users who have their account deleted will set all future attendances to 'Flake'
  * They won't receive a notification for any flakiness
  * All their tea times etc. will be preserved
  * Their account can be restored
* Tea Times and Attendances are never deleted. They can be restored. They will not show up in anything without chaining `unscoped` on the query

# KNOWN ISSUES
~~* A user who has their account deleted will be unable to reregister for TWS w.o Admin Intervention until we do feature work to enable account restoration~~