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