email_notification: Include prior message context only when mentioned.

Earlier, email message notifications included prior messages sent
to the same topic for context. This is more confusing than helpful
for messages that the user is likely to have received notifications
for all the prior messages in the conversation already (or read them
in the Zulip UI).

Now, we include prior context only when the user is mentioned via
personal, group, stream or topic wildcard mention.

Fixes #27479.

zerver/models.py

@@ -3662,6 +3662,20 @@ class UserMessage(AbstractUserMessage):
         """
         return UserMessage.objects.select_for_update().order_by("message_id")
 
+    @staticmethod
+    def has_any_mentions(user_profile_id: int, message_id: int) -> bool:
+        # The query uses the 'zerver_usermessage_any_mentioned_message_id' index.
+        return UserMessage.objects.filter(
+            Q(
+                flags__andnz=UserMessage.flags.mentioned.mask
+                | UserMessage.flags.wildcard_mentioned.mask
+                | UserMessage.flags.topic_wildcard_mentioned.mask
+                | UserMessage.flags.group_mentioned.mask
+            ),
+            user_profile_id=user_profile_id,
+            message_id=message_id,
+        ).exists()
+
 
 def get_usermessage_by_message_id(
     user_profile: UserProfile, message_id: int