Subscription status fix

Here are the changes I made (I haven't added tests, sorry!):
- models.py: assignment of current_subscription.canceled_at in Customer.cancel_subscription(),
- views.py: slight change in message info in CancelSubscriptionView when status is not canceled,
- \_subscription_status.html: added the case where subscription has been canceled but is active until period_end,
- cancel_subscription.html: if subscription has already been canceled, the view does not show "Cancel my subscription" but a simpler content.