Move db.sql.util to db.util

Now that sqlalchemy is providing the database abstractions, there is no
need to hide everything in db.sql. sub-modules. It complicates the code
and provides a futher layer of indirection.

Move the db.sql.util.py to db.util.py and adapt the importers.

mediagoblin/tests/test_sql_migrations.py

@@ -26,7 +26,7 @@ from sqlalchemy.sql import select, insert
 from migrate import changeset
 
 from mediagoblin.db.sql.base import GMGTableBase
-from mediagoblin.db.sql.util import MigrationManager, RegisterMigration
+from mediagoblin.db.util import MigrationManager, RegisterMigration
 from mediagoblin.tools.common import CollectingPrinter