report appeals for extensions to a different queue

[eviljeff]

Fixes: mozilla/addons#15161

Description

Route appeals for decisions on extensions (that were made in Cinder) to T&S's escalation queue, rather than the original listings queue.

Context

There's some strictly unnecessary refactoring of get_entity_helper and CinderAddon from an earlier version of the patch when I went a different direction but I've kept it because it's pretty small and I like it better.

Testing

Like most cinder related patches, it's a bit of pain to set up fully. To get to the state where you can test this you need:

• to have an abuse report reported to Cinder;
• then a decision made on the job for that abuse report in Cinder;
• and replay the webhook request from the decision locally with curl, etc.
  • (It is possible to fake the webhook response by manually creating the ContentDecision, or updating another ContentDecision instance, but it's out of scope for this testing detail to explain exactly how 😛)

To test the patch itself:

• Once you have a decision from Cinder locally, check the email in fakemail for the appeal link (or compose the url yourself if you know how).
• Submit the appeal.
• See in Cinder the appeal is in the amo-escalations ("T&S Escalations") queue, rather than in the AMO dev Listings queue.

Bonus testing would be to repeat all of this with a theme and a non-add-on content type (e.g. collection) to check those go into their original queues in Cinder; plus appeals on reviewer tools decisions also go to their original (amo dev addon infringement) queue.

Checklist

• Add #ISSUENUM at the top of your PR to an existing open issue in the mozilla/addons repository.
• Successfully verified the change locally.
• The change is covered by automated tests, or otherwise indicated why doing so is unnecessary/impossible.
• Add before and after screenshots (Only for changes that impact the UI).
• Add or update relevant docs reflecting the changes made.