`@lti` decorator depends on `app` rethink if dependency can be removed

[iceraj]

Not all frameworks will have app object. Rethink if we can get config in some other way and refactor the code so it does not require app parameter.

[carsongee]

Just to add to this, app and app.config is not available in blue prints, so this will really needs to do current_app or similar to be used in blueprints