Add "Mailing List" users

Summary:
Ref T8387. Adds new mailing list users.

This doesn't migrate anything yet. I also need to update the "Email Addresses" panel to let administrators change the list address.

Test Plan:
  - Created and edited a mailing list user.
  - Viewed profile.
  - Viewed People list.
  - Searched for lists / nonlists.
  - Grepped for all uses of `getIsDisabled()` / `getIsSystemAgent()` and added relevant corresponding behaviors.
  - Hit the web/api/ssh session blocks.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: eadler, tycho.tatitscheff, epriestley

Maniphest Tasks: T8387

Differential Revision: https://secure.phabricator.com/D13123

resources/sql/autopatches/20150602.mlist.1.sql

@@ -0,0 +1,2 @@
+ALTER TABLE {$NAMESPACE}_user.user
+  ADD isMailingList BOOL NOT NULL;