Fix things for MySQL InnoDB users

The OpenStack project is working on an askbot that will run on top of MySQL with InnoDB. To that end, getting the migrations to work winds up being important. Most of the problems encountered came from things getting run multiple-times... these patches should fix that.

In the process, I also cleaned up the false errors in the migrations so that scanning for real problems was easier. They were all due to migrations trying to create columns that had already been created in the initial syncdb.