Add a policy restricting mailing list management
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
Maniphest Tasks: T7291
Differential Revision: https://secure.phabricator.com/D11783