Move all the migration tools into new migration_tools.py

Factor all the migration related stuff out into a new
.db.sql.migration_tools.
First we don't have to load this module for our normal
server.
Second it makes all the import dependencies a little more
cleaner.

mediagoblin/gmg_commands/dbupdate.py

@@ -19,7 +19,7 @@ import logging
 from sqlalchemy.orm import sessionmaker
 
 from mediagoblin.db.open import setup_connection_and_db_from_config
-from mediagoblin.db.util import MigrationManager
+from mediagoblin.db.sql.migration_tools import MigrationManager
 from mediagoblin.init import setup_global_and_app_config
 from mediagoblin.tools.common import import_component