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