Change esp.formstack to use esp.cache.cache_function

Also tweak the API a little bit to be a little more consistent with
other parts of the ESP codebase; mostly changing .get_x() -> .x().
Remove obsolete functions in esp.application
Tweak behavior of FormstackStudentApp.objects.fetch

-- Use cache_function
-- Use a locking mechanism to prevent infinite recursion
-- Always save the model instances to the database

Also tweak FormstackAppSettings.create_username_field to always save
the username field id to the database.
Improve admin panel interface for FormstackAppSettings