feature/responsive preview

responsiveness for preview 

- [x] finish the code
- [x] gather feedback for my changes
- [x] submit changes to the documentation


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           | none

merge following PR before https://github.com/sulu-cmf/SuluAdminBundle/pull/94