Add a policy restricting mailing list management

Summary:
Fixes T7291. There are a class of spam/annoyance attacks here that we should be more strict about preventing, since you can add an individual's address as a mailing list.

This application is likely on the way out so I didn't bother trying to do per-object policies.

Test Plan: Set policy restrictively and could no longer create or edit mailing lists.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7291

Differential Revision: https://secure.phabricator.com/D11783