[10.x] Ability to get User by stripe id

[edalzell]

There was no convenient way to grab the relevant user from a webhook event (like customer.subscription.created without copying the code from WebhookController.

Move that method to the Cashier class and update the calls in WebhookController.

Now in my webhook controller I can do:

$user = Cashier::findBillable($payload['data']['object']['id']);

Updated previous PR as per Dries' comments in #867.

[driesvints]

Looks good! Thanks :)