#22340  -- Moved more DDL to schema editors

This series of commit move part of DatabaseCreation work to schema editors. The main achievement here is to make migrate use schema editor for syncing unmigrated apps.
Add to release notes? Deprecation timeline may need to be edited. It says, "The DatabaseCreation class on each database backend will be removed, and all table/schema editing will be moved to be via SchemaEditor instead." Do you think that's going to be the final endpoint?