Expose more detail about enforcement actions with delays

[eviljeff]

Fixes mozilla/addons#16305

Description

Expose the ActionClass description in the reviewer tools, which is more verbose and includes delay delays, rather than the enforcement action label.

Context

Also changed all the tenses on the description properties to future tense (including all the none Add-on action classes for consistency, even though they aren't displayed anywhere). Follow-up actions didn't need the description (re)defining - we were already doing it to expose in the emails to developers.

Testing

• enable enable-policy-review-selection waffle switch
• expose a selection of policies in the reviewer tools via django admin
  • at least one with follow-up actions - amo-fu-*
  • at least one with amo-addon-version-rejection-warning
• select policies in the reviewer tools to see the enforcement action labels

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.