This is effectively a step closer to what was proposed in
https://github.com/zulip/zulip/pull/18678#discussion_r644490540 when
this code was written in #18678.
If the Customer object has neither of a Stripe id, nor any historical
plans, then there's no real billing association contained in the
existence of the Customer object, and it's safe to delete.
By registering a post_delete handler to clear appropriate caches in a
nicer way, we can get rid of the ugly flush-memcached call in the
delete_realm command.
Anders Kaseorg
Alex Vandiver
Tim Abbott
Mateusz Mandera