Updates so that dbupdate command works

- Various fixes to dbupdate itself
 - Switching db/sql/migrations.py to use a dict instead of a list
 - Registering the function

mediagoblin/db/sql/migrations.py

@@ -14,4 +14,4 @@
 # 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/>.
 
-MIGRATIONS = []
+MIGRATIONS = {}