push_notifications: Clear push notifications on message deletion.

This commit adds support to revoke mobile push notifications
for messages when deleted.

Fixes #26584.

Signed-off-by: Prakhar Pratyush <prakhar@zulip.com>

zerver/models/messages.py

@@ -642,6 +642,13 @@ class UserMessage(AbstractUserMessage):
                 ),
                 name="zerver_usermessage_active_mobile_push_notification_id",
             ),
+            models.Index(
+                "message",
+                condition=Q(
+                    flags__andnz=AbstractUserMessage.flags.active_mobile_push_notification.mask
+                ),
+                name="zerver_usermessage_message_active_mobile_push_notification_idx",
+            ),
         ]
 
     @override