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

scripts/ssh/ssh-exec.php

@@ -182,11 +182,11 @@ try {
       'P' => $user->getPHID(),
     ));
 
-  if (!$user->isUserActivated()) {
+  if (!$user->canEstablishSSHSessions()) {
     throw new Exception(
       pht(
-        'Your account ("%s") is not activated. Visit the web interface '.
-        'for more information.',
+        'Your account ("%s") does not have permission to establish SSH '.
+        'sessions. Visit the web interface for more information.',
         $user->getUsername()));
   }