FEATURE: Allow users to disable new PMs.

https://meta.discourse.org/t/is-it-possible-to-disable-private-messaging-for-a-specific-user/46391

app/services/user_updater.rb

@@ -34,7 +34,8 @@ class UserUpdater
     :email_in_reply_to,
     :like_notification_frequency,
     :include_tl0_in_digests,
-    :theme_key
+    :theme_key,
+    :allow_private_messages,
   ]
 
   def initialize(actor, user)