Notify Legal on Takedown Action

[chrstinalin]

Fixes mozilla/addons#16268

Description

Members of Legal-Takedown-Notifications are notified by email when a takedown:

1. Has been paused and put in second level approval, or
2. Has been executed.

Testing

See: mozilla/addons#16052 (comment)

Locally

1. Add account(s) to the Legal-Takedown-Notifications access group.
2. Use ./manage.py fake_cinder_webhook to send a mock amo-legal-disable-addon payload for:

• A promoted add-on
• A regular add-on

4. An email to Legal-Takedown-Notifications emails should be present in admin fakemail.

dev/stage

1. Add account(s) to the Legal-Takedown-Notifications access group.
2. Through Cinder, trigger a legal takedown for:

• A promoted add-on, causing it to be put in second level approval, and
• A regular add-on, causing the takedown to occur.

6. An email to Legal-Takedown-Notifications emails should be present in admin fakemail.

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.