Change codebase to query or create correct User model

The code base had many references to User.username and other
specific to LocalUser attributes as that was the way it use to exist.
This updates those to query on the generic User model but filtering
by attributes on the LocalUser.

mediagoblin/plugins/archivalook/tools.py

@@ -13,7 +13,6 @@
 #
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
-
 import six
 
 from mediagoblin.db.models import MediaEntry, User, LocalUser