Return JsonResponse objects instead of HTML responses

Fixes https://github.com/sulu-cmf/sulu-standard/issues/318

__Tasks:__

- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/318
| BC Breaks     | 
| Doc           |