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Bug Fixes
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* Adding missing methods to ``SubscribeView`` and ``Manager`` to record
payment transactions. Added additional method
(``retrieve_transaction_date``) to help with transaction date
specification. Reworked method calls around payment processing to
streamline passing of arguments between functions to reduce need to
override methods.
* Fixing issue in ``Manager`` class where the future billing date was
based off the current datetime, rather than the last billed datetime.
* Adding method to update next billing datetimes for due subscriptions
in the ``Manager`` class.
* Switching the default ``success_url`` for ``SubscribeView`` and
``CancelView`` to the user-specific list of their subscriptions,
rather than the subscription CRUD dashboard.