link the activity log to the ContentDecision so we can backtrack it

[eviljeff]

Fixes: mozilla/addons#15228

Description

Adds an intermediary ActivityLog model that allows us to track ContentDecision that are linked to a reviewer/moderator activity.

Most of the change here is to enable all actions from Cinder to generate an ActivityLog and link the decision to it. The change in notify_reviewer_decision, that is called from the reviewer tools, is more of an intermediate hack because the activity log was already created - it's going to need more refactoring to create the ContentDecision earlier that will be addressed in other issues.

Context

Having a link between the ContentDecision and the connected activity will unlock more accurate appeal handling (we can flag the exact version that was rejected, rather than guessing at the add-on level); and the 2nd level approvals for reviewer tools actions. (And potentially drop CinderJobLog and/or CinderPolicyLog as the ContentDecision already links to jobs and policies)

Testing

Difficult to test functionally because it's a dependency for further work, (on top of the usual fun for testing Cinder integration).

To test Cinder actions:

• Enable all the Cinder/DSA related waffle switches
• Report an add-on (or other content) to Cinder
• Resolve the job in Cinder with a policy that takes down the content
• replay the webhook payload locally
• See the content is deleted/disabled
• Check the latest ActivityLog (it should be the latest, but you may have to dig a little) and see it has a .contentdecisionlog_set manager with 1 ContentDecisionLog instance, where the .decision points to the created CinderDecision.

Bonus testing with appeals:

• appeal the takedown as the content owner (e.g. add-on developer)
• grant the appeal in Cinder (for addons the appeal will be in the T&S Escalations queue)
• replay the webhook payload locally
• see the content is re-enabled
• Check the latest ActivityLog (it should be the latest) and see it has a .contentdecisionlog_set manager with 1 ContentDecisionLog instance, where the .decision points to the created CinderDecision.

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.