Payments: Move Stripe logic to the form
Add decorator for enabling/disabling Sylva features
Payments: Update models
Payments: Add migration
Payments: Update variable name
Payments: refactoring
Payments: new logic for subscription/unsubscription

Let the user create several subscriptions, thought, some restrictions will
be added in next commits.

Override delete() method from StripeCustomer in order to do some checks with
Accounts.
Payments: Add subscription/unsubscription restrictions