Subscriptions: remove old code (#10642)
* Subscriptions: remove old code
We are now fully using dj-stripe for subscriptions, so we can our
code for that. I'm not fully removing the app yet,
so we can do a remove the old tables with zero downtime.
* Tests
* Format
* Fix migration
* Fixes
* Format
* Don't create subscription when checking for feature
* Include requirements in cache key
* Check for created subscriptions
* Updates from review
* Merge subscription.created and checkout.completed events
* Don't include data migration, make this a deploy step instead
* Fix test
