R1:992c19957760

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…