users: Send user remove events on user deactivation.

Guests might lose access to deactivated users if the user
is not involved in any DM with guest. This commit adds
code to send "realm_user/remove" events for such cases.

zerver/lib/events.py

@@ -996,6 +996,12 @@ def apply_event(
                     user_profile.realm, person_user_id
                 )
                 state["raw_users"][person_user_id] = inaccessible_user_dict
+
+            if include_subscribers:
+                for sub in state["subscriptions"]:
+                    sub["subscribers"] = [
+                        user_id for user_id in sub["subscribers"] if user_id != person_user_id
+                    ]
         else:
             raise AssertionError("Unexpected event type {type}/{op}".format(**event))
     elif event["type"] == "realm_bot":