Subscriptions: use djstripe events to mail owners

[stsewd]

Instead of trying to replace the existing queries, I'm making use of djstripe events/webhooks.

• Instead of a daily task to email users with a canceled subscription,
  we use a webhook to email users the exact time the subscription changes to "canceled" (this is the moment the subs ends, not when a user cancels a subscription, since it will remain active till the end of the cicle).
• Instead of querying organizations with subs in trialing, we query the subs directly,
  and get the organization from them.

With this, we don't depend on having just one subs per-organization, and also stop relying on our models a little less.
The queries that are used to disable orgs aren't migrated yet since they query canceled subscriptions (#9652 (comment)).

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--9661.org.readthedocs.build/en/9661/

• Developer's documentation (dev): https://dev--9661.org.readthedocs.build/en/9661/

[stsewd]

we were requiring passing this parameter, and always using HttpRequest when we didn't have it, so looks like a good default. Doesn't look like we are using it for making decisions.

[stsewd]

this can also be a collection of helpers, but using a queryset we don't need to import those and just use the model directly :)

[humitos]

Looks good! This is pretty close to get merged.

Since we are changing how most of our email notification works, we will need to QA this closely when deploying these changes to make sure they keep continue working correctly. In particular that we are handling the Stripe webhooks correctly and also the Celery tasks.