[FEATURE] Content Type media selection

Content Type image selection:

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation
- [x] content type and container
- [x] service configuration
- [x] template XML / TWIG (https://github.com/sulu-cmf/sulu-standard/pull/90)
- [x] javascript integration
- [x] datagrid group (https://github.com/sulu-cmf/SuluAdminBundle/pull/121)
- [x] datagrid preselected (https://github.com/massiveart/husky/pull/319)

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | url to commit
@wachterjohannes Do we need these commented lines?