Changed migrations specs to require that migration_stub calls exist.

Before, using migration_stub could reference a global model.  The goal
of migration_stub was to prevent loading global models so that we can
ensure that at the least tests enforced that rule.  Now, migration_stub
enforces the rule.

This exposed many places where migration_stub was being used incorrectly
or where the migration itself incorrectly referenced global models.

In addition, 2 migrations weren't testing anything except pure
ActiveMigration code, which isn't worth doing, so they were removed.

@jrafanie Please review.
Is this the one that ensures the global model isn't loaded?  Where does it yell at us if we do load a global model?

If it makes sense, perhaps we delete the useless specs in one commit, cleanup any formatting in another, fix the reserved stub in another, fix the remaining global models in one last commit...

That should make it easier to follow.