expose followup actions with policies in emails

[eviljeff]

Fixes mozilla/addons#16196

Description

Changes the takedown email to reference follow-up actions, if any. Plus now an email is sent when the block takes effect.

Context

Tried to keep it as generic as possible - and I'm intending to re-use description_with_eta in the reviewer tools

Testing

• In Cinder, takedown an add-on with a policy that has follow-up actions
• see the email mentions the follow-up actions
• with policy enforcement enabled in the reviewer tools it should work from there too
• once the BlocklistSubmission has been processed (edit in django admin to remove the delay, unless you want to wait x days) there should be another email that informs the developer of the block

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.