This hardcoded the db_alias fetched from schema_editor and forces django
to try and migrate any second database you use, rather than routing to
the default database. In testing a build from scratch, these do not
appear needed.
Using using() prevents us from using multiple databases behind edxapp.
Additionally - add back a removed backwards migration from certificates
0003. I have no idea why this was dropped in the 1.8 upgrade.
