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.