CRUD for PromotedGroup via django admin

[eviljeff]

Fixes mozilla/addons#16342

Description

Enables django admin create, update, and delete for PromotedGroup instances so add-ons Operations (well, admins) can manage PromotedGroups themselves. Triggers elasticsearch reindexes if changes are made to public groups.

Context

Changing badged groups isn't possible unless/until we resolve mozilla/addons#16341

I did consider adding further guard-rails for public groups - especially badged groups - because frontend is going exhibit changes immediately if you do something such as changing api_name, but left it at a warning in the field label instead. It's complicated - api_names such as spotlight have side-effects (e.g. warning being removed) that we can't account for in addons-server, and notable isn't even a public group and it's api_name is important. For now, it's locked down to *:* anyway, but especially if it's opened up to users with narrower permissions we should reconsider more guard-rails.

Testing

• edit a promoted group in http://olympia.test/admin/models/discovery/discoverypromotedgroup/
• create a new group and/or delete a group
• to test the reindexing is triggered you can:
  • select/create a promoted group
  • add an add-on to the group in http://olympia.test/admin/models/discovery/discoveryaddon/
  • either update with is_public=True or update the api_name of the grou
  • see the promoted group is correctly exposed in http://olympia.test/api/v5/addons/search/?q=<whatever the add-on is named>

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.