Read Promoted Group Information Via Models

[chrstinalin]

Fixes: mozilla/addons#15350

Description

Reads promoted group information via models.

Testing

Promoted add-on behaviour should be largely the same (as this PR is only switching the read behaviour, and not explicitly allowing multiple promoted addons.), except:

• v3 and v4 API should directly return a single promoted group. v5 returns a list (This is supported by frontend via Frontend Compatibility for Multiple Promoted Addon Groups addons-frontend#13343).

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 or update relevant docs reflecting the changes made.

[KevinMind]

A few questions and wondering if it would be possible to split this into 2 PRs. This PR should be able to move the "read" operations from PromotedAddon to PromotedAddonPromotion without otherwise changing much of anything like function signatures.

Then we would migrate the "write" operations from PromotedAddon to PromotedAddonPromotion.

Finally, you can modify the method signatures to support multiple promoted groups.

I think it might be easier/safer to get this out in these smaller PRs that focus on one specific part.

[chrstinalin]

A few questions and wondering if it would be possible to split this into 2 PRs. This PR should be able to move the "read" operations from PromotedAddon to PromotedAddonPromotion without otherwise changing much of anything like function signatures.

I think its at the point where splitting it further would make things more difficult. All the changes here are pretty interconnected at this point, the signals part was the only part I could manage to split off (and even then it wasn't much).

[KevinMind]

I have a strong-ish opinion about the API changes. Modifying the signature and source of truth from the Promotion to the Group is not really what I thought we were going to do and I think it was better the way it was.

Let's discuss on a call and figure out what to do and move from there.

[KevinMind]

Approved, fix the tests and make sure to notify in the next release notes that we need QA to do a full test of promoted addon behavior.