Move InvalidId to db.util

Instead of import InvalidId from the low level bson module,
first import it in our db.util wrapper and second import it
from pymongo.errors.

mediagoblin/db/util.py

@@ -17,4 +17,5 @@
 
 # Imports that other modules might use
 from pymongo import DESCENDING
+from pymongo.errors import InvalidId
 from mongokit import ObjectId