Allow appeals to be accepted with override

[eviljeff]

Fixes mozilla/addons#16241

Description

Adds a "Resolve appeals with Override" reviewer tools action, that specifically handles overriding developer appeals with a policy with a non-negative enforcement action (e.g. Approve).

Context

• "Resolve with Denial" is the existing action, that only needed a minor change, to defer executing the action to the ActionClass, so we send out the correct email(s).
• "Override as accept" appeal is what Cinder does, so I followed the same pattern here - and keeping it as a specific action means we can be deliberate about what happens, and ensure the versions being appealed are correctly reversed.
• Cinder's appeal UI allows an override to any policy, including other negative policies, which is generally fine because all of the negative policies involve taking the add-on down entirely. In the reviewer tools though, it could be problematic - if, for example, a reviewer resolved a developer appeal by overriding to a policy that did a different combination of rejecting/disabling/follow-up actions, we'd need to handle all the possible combinations. Instead, I'm only offering non-negative policies. When we do [Task]: Have reviewers override a decision addons#16242 we may need to handle those scenarios - but at least it will be a deliberate decision on the part of the reviewer.
• Because an appeal reverses the action previously taken, if a version awaiting approval was rejected, accepting the appeal with Approve would change it back to awaiting approval - not approve it. Which I think makes conceptual sense(?), but also may be unexpected from a reviewer POV. Actually approving versions will have to be done with [Task]: Have reviewers select a policy rather than an action, for non-negative actions addons#16239 anyway.
• the jobs selector is particularly unhelpful for appeals, so I filed [Task]: Improve jobs UI in reviewer tools for appeals addons#16266 to improve

Testing

• Make a decision with a policy that has some negative enforcement actions (e.g. reject, with a follow-up)
• appeal that decision
• use "Resolve appeals with Override" to accept that appeal in the reviewer tools
• see the action was reversed (version was unrejected, back to the prior file status) and any follow-ups similarly cancelled/reversed

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.