models: Manage index from migration 0095 with Django.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/models.py

@@ -3128,6 +3128,12 @@ class UserMessage(AbstractUserMessage):
 
     class Meta(AbstractUserMessage.Meta):
         indexes = [
+            models.Index(
+                "user_profile",
+                "message",
+                condition=Q(flags__andz=AbstractUserMessage.flags.read.mask),
+                name="zerver_usermessage_unread_message_id",
+            ),
             models.Index(
                 "user_profile",
                 "message",