settings: Add support for overriding APNS_TOPIC and ZULIP_IOS_APP_ID.

Documentation added by tabbott.

docs/production/mobile-push-notifications.md

@@ -228,6 +228,8 @@ push notifications through the new app is quite straightforward:
   from Apple's developer console.  Set `APNS_SANDBOX=False` and
   `APNS_CERT_FILE` to be the path of your APNS certificate file in
   `/etc/zulip/settings.py`.
+* Set the `APNS_TOPIC` and `ZULIP_IOS_APP_ID` settings to the ID for
+  your app (for the official Zulip apps, they are both `org.zulip.Zulip`).
 * Restart the Zulip server.
 
 [apple-docs]: https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/APNSOverview.html

zerver/lib/push_notifications.py

@@ -137,7 +137,7 @@ def send_apple_push_notification(user_id: int, devices: List[DeviceToken],
         def attempt_send() -> Optional[str]:
             try:
                 stream_id = client.send_notification_async(
-                    device.token, payload, topic='org.zulip.Zulip',
+                    device.token, payload, topic=settings.APNS_TOPIC,
                     expiration=expiration)
                 return client.get_notification_result(stream_id)
             except HTTP20Error as e:

zproject/settings.py

@@ -343,9 +343,13 @@ DEFAULT_SETTINGS.update({
     # this is disabled in production, but we need it in development.
     'POST_MIGRATION_CACHE_FLUSHING': False,
 
-    # Settings for APNS.  Only needed on push.zulipchat.com.
+    # Settings for APNS.  Only needed on push.zulipchat.com or if
+    # rebuilding the mobile app with a different push notifications
+    # server.
     'APNS_CERT_FILE': None,
     'APNS_SANDBOX': True,
+    'APNS_TOPIC': 'org.zulip.Zulip',
+    'ZULIP_IOS_APP_ID': 'org.zulip.Zulip',
 
     # Max number of "remove notification" FCM/GCM messages to send separately
     # in one burst; the rest are batched.  Older clients ignore the batched
@@ -1270,9 +1274,6 @@ LOGIN_REDIRECT_URL = '/'
 # (HEARTBEAT_MIN_FREQ_SECS + 10)
 POLL_TIMEOUT = 90 * 1000
 
-# iOS App IDs
-ZULIP_IOS_APP_ID = 'org.zulip.Zulip'
-
 ########################################################################
 # SSO AND LDAP SETTINGS
 ########################################################################