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/lib/server_initialization.py

@@ -56,6 +56,8 @@ def create_internal_realm() -> None:
     # just ensures these get low IDs in production, and in development
     # avoids an extra database write for the first HTTP request in
     # most tests.
+    #
+    # These are currently also present in DATA_IMPORT_CLIENTS.
     get_client("Internal")
     get_client("website")
     get_client("ZulipMobile")