Add ability to run db:abort_if_pending_migrations and db:test:prepare from the app namespace.

The problem is that after adding `require 'parallel_tests/tasks'` to my engine's rakefile I still can't run `rake parallel:prepare` as standard `load 'rails/tasks/engine.rake'` doesn't bring `db:abort_if_pending_migrations` and `db:test:preapre` to the global namespace, so they are located at the `app:` namespace.