migration: Rename 'populate_db' Client to 'ZulipDataImport'.

This commit:

* Creates a migration to rename any existing Client with
name="populate_db" to "ZulipDataImport".
* Updates populate_db.py to use ZulipDataImport for new
message creation

These changes should make code to identify imported messages
considerably more readable.

Fixes #33909.

zerver/tests/test_slack_importer.py

@@ -1017,6 +1017,7 @@ class SlackImporter(ZulipTestCase):
         ) = slack_workspace_to_realm(
             "testdomain", realm_id, user_list, "test-realm", "./random_path", {}
         )
+
         test_zerver_realmdomain = [
             {"realm": realm_id, "allow_subdomains": False, "domain": "testdomain", "id": realm_id}
         ]