push_notifs: Make appid required on add_apns_device_token.

We're going to need to use this information, so we shouldn't just
assume a value; the client should tell us the actual value.

Conveniently, the Zulip mobile app does already pass this parameter
and has since forever.  So we can just start requiring it, with no
compatibility constraint.

zerver/views/push_notifications.py

@@ -1,6 +1,5 @@
 from typing import Optional
 
-from django.conf import settings
 from django.http import HttpRequest, HttpResponse
 from django.utils.translation import gettext as _
 
@@ -35,7 +34,7 @@ def add_apns_device_token(
     request: HttpRequest,
     user_profile: UserProfile,
     token: str = REQ(),
-    appid: str = REQ(default=settings.ZULIP_IOS_APP_ID),
+    appid: str = REQ(),
 ) -> HttpResponse:
     validate_token(token, PushDeviceToken.APNS)
     add_push_device_token(user_profile, token, PushDeviceToken.APNS, ios_app_id=appid)