Extract get_turtle_message().

This seems like kind of a silly function to extract
to topic.py, but it will theoretically help us sweep
"subject" if we change the DB.

It had test coverage.

tools/linter_lib/custom_check.py

@@ -40,7 +40,6 @@ FILES_WITH_LEGACY_SUBJECT = {
     # TRY TO FIX THESE! If you can't fix them, try to
     # add comments here and/or in the file itself about
     # why sweeping subject is tricky.
-    'zerver/lib/onboarding.py',
     'zerver/lib/stream_topic.py',
 
     # This has lots of query data embedded, so it's hard

zerver/lib/onboarding.py

@@ -5,6 +5,7 @@ from zerver.lib.actions import set_default_streams, bulk_add_subscriptions, \
     internal_prep_stream_message, internal_send_private_message, \
     create_stream_if_needed, create_streams_if_needed, do_send_messages, \
     do_add_reaction_legacy, create_users, missing_any_realm_internal_bots
+from zerver.lib.topic import get_turtle_message
 from zerver.models import Realm, UserProfile, Message, Reaction, get_system_bot
 
 from typing import Any, Dict, List, Mapping
@@ -114,8 +115,5 @@ def send_initial_realm_messages(realm: Realm) -> None:
     # We find the one of our just-sent messages with turtle.png in it,
     # and react to it.  This is a bit hacky, but works and is kinda a
     # 1-off thing.
-    turtle_message = Message.objects.get(
-        id__in=message_ids,
-        subject='topic demonstration',
-        content__icontains='cute/turtle.png')
+    turtle_message = get_turtle_message(message_ids=message_ids)
     do_add_reaction_legacy(welcome_bot, turtle_message, 'turtle')

zerver/lib/topic.py

@@ -227,3 +227,12 @@ def get_topic_history_for_web_public_stream(recipient: Recipient) -> List[Dict[s
     cursor.close()
 
     return generate_topic_history_from_db_rows(rows)
+
+def get_turtle_message(message_ids: List[int]) -> Message:
+    # This is used for onboarding, and it's only extracted
+    # here to make subject -> topic sweeping easier.
+    turtle_message = Message.objects.get(  # nolint
+        id__in=message_ids,
+        subject='topic demonstration',
+        content__icontains='cute/turtle.png')
+    return turtle_message