Add check for deleted user in subscription deleted Stripe web hook
event handler, and equivalent test case.
Add logging for web hook 500 errors to aid future debugging.
Admins can generate a prepaid code, which a user can use to subscribe
for free via the account/subscription page.
The subscription will be identical to the normal monthly subscription
(e.g. 3500 gems per month), except they won’t be charged.
Does not require the recipient to enter billing information.
Can be applied to an existing subscription, which will be converted to
free.
Prepaid code can only be used once.
Prepaid subscription cannot be unsubscribed via the UI.
