Use application PHIDs for mailing lists

Summary:
Ref T2715. Ref T603. Ref T2625.

• Implement policies.
• Use policy queries.
• Use ApplicationSearch.
• Use application PHIDs.

Test Plan: Browsed things with lists CC'd; edited lists; created a list, used phid.query to query handles.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603, T2625, T2715

Differential Revision: https://secure.phabricator.com/D6513