user-status: Update presence_enabled with changes to user status away.

When a user toggles a status update for `away=True|False`, we now update
their `presence_enabled` setting to match (`away!=presence_enabled`).

First step of making user status `away` updates a deprecated way to
access presence_enabled for clients supporting older servers, and
checkpoint commit before migrating users with a current UserStatus
of `status=AWAY` to have their `presence_enabled` set to `False`.

Note that when user status `away` is updated, we now send 4 events:
user_status, user_settings, presence, and update_global_notifications.

Also, this means that these updates change the UserPresence.status
value, which impacts the test for importing and exporting user
information.

Part of transitioning from 'unavailable' user status feature to
'invisible mode' user presence feature.

zerver/actions/user_status.py

@@ -1,5 +1,6 @@
 from typing import Optional
 
+from zerver.actions.user_settings import do_change_user_setting
 from zerver.lib.user_status import update_user_status
 from zerver.models import UserProfile, UserStatus, active_user_ids
 from zerver.tornado.django_api import send_event
@@ -14,6 +15,13 @@ def do_update_user_status(
     emoji_code: Optional[str],
     reaction_type: Optional[str],
 ) -> None:
+    # Deprecated way for clients to access the user's `presence_enabled`
+    # setting, with away != presence_enabled.
+    if away is not None:
+        user_setting = "presence_enabled"
+        value = not away
+        do_change_user_setting(user_profile, user_setting, value, acting_user=user_profile)
+
     if away is None:
         status = None
     elif away: