add allow_reasons for reject_listing_content action

[eviljeff]

Fixes: mozilla/addons#16101

Description

Fixes a bug when reject listing content is enabled in reviewer tools, but cinder policy selection isn't enabled. Review Reasons weren't available for selection, but the action required a reason to be selected.

Context

A known problem with having new features enabled on addons-dev for months, but not on stage/prod ...

Testing

• set waffle switches:
  • enable-content-rejection to True
  • `cinder_policy_review_reasons_enabled to False
• try to content reject an addon. The action should show review reasons (you may need to add one or more in django admin), and require at least one to be selected.
• the content rejection should work

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.