Load content twig extension

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/328
| BC Breaks     | none
| Doc           | https://github.com/sulu-cmf/docs/commit/aa59b86e0e0fa279775aacb40dcadf2e998021ad and https://github.com/sulu-cmf/docs/commit/adab368881c24f99f482efe92f912b139a0c0f6d