🎨 Added staff notification when a sub is canceled due to failed payments

[sagzy]

ref https://linear.app/tryghost/issue/ENG-1254

• when a subscription is canceled automatically by Stripe (e.g. due to multiple failed payments), we now send a staff notification
• logic before: if a member cancels a sub in Portal, then send a staff notification
• logic now: if a subscription was active, but is now set to cancel immediately or at the end of the billing period, then send a staff notification.
• with that logic change, we now send a cancellation staff notification when:
  1. A member cancels their sub in Portal (existing)
  2. A staff member cancels a member sub in Stripe (new)
  3. A staff member cancels a member sub in Admin (new)
  4. A sub is canceled automatically by Stripe because of multiple failed payments (new)
• the copy of the staff notification email has also been updated to take into account 1) manual vs automatic cancellations, and 2) immediate vs end of billing period cancellations

[sagzy]

Pending copy changes on the email notification (discussion with Peter here)