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.
> 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...

I disagree.  I'm of the opinion that each commit should be able to live on it's own *and* pass all the specs.  Changing code and specs in 2 different commits separates the "why".  When someone goes back and searches in history, how will they know why we, for example, deleted the useless specs.