Rename global app objects to flask_app/celery_app

hapi-flask and hapi-worker both have global variables called 'app'. In
the former case, this is an instance of the Flask class, and in the
latter, its an instance of the Celery class.

To avoid confusion, and prepare for the upcoming transition to
HALApplication, rename these to flask_app and celery_app, respectively.
This also makes it clear when reading the code which is bring referred
to.