Immediate cancelation of canceled memberships

[bkgoodman]

Memberships are privy to a 2-week "grace period" when people's subscriptions expire. This is to give people warning before cutting them off when things happen like on-file credit cards expire and need to be changed.

However, this grace period currently also applies to canceled memberships. Furthermore, since the system only looks at the "end date" of the subscription, and then grants the 2-week "grace period" on top of that - someone could cancel a subscription [near beginning of month] - and have access up to 6 weeks later.

We should maybe:

1. Treat canceled as different than past_due
2. Use a non-null canceled_at field as a definitive cutoff.