Patch Subscription

Every refusal here is a way the console could make Admiral's idea of a plan disagree with the bill, or hand out access with no end.

• active and past_due come only from the payment provider's webhook (models/billing.py: "the provider webhook is the only writer of status"), so they cannot be typed in.
• While a provider bills the plan, its tier, status and dates are the provider's: changing any of them here would make entitlements disagree with what the customer is charged, and cancelling would cut access while the provider keeps billing.
• Complimentary access needs an end date. Without one it used to keep the old trial end, and resolve() turned it into cancelled the day the trial would have ended, or at once if that day had passed.