[FEATURE] node properties (external / internal link)

node properties (external / internal link):

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation
- [x] update config, templates (https://github.com/sulu-cmf/sulu-standard/pull/101)
- [x] structure node types (https://github.com/sulu-cmf/sulu/pull/123)
- [x] async header init (https://github.com/sulu-cmf/SuluAdminBundle/pull/126)
- [x] generate right urls (https://github.com/sulu-cmf/SuluWebsiteBundle/pull/23)
- [x] change template from non content to content type
  * error because internal- / external-link template is hidden
- [x] css adaptions
- [x] translations
- [x] some other things i don´t know (=


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | no => https://github.com/sulu-cmf/sulu/pull/123
| Fixed tickets | none
| Doc           | https://github.com/sulu-cmf/docs/blob/master/detail-specification/300-webspaces/DET-312-Node-Type.md and https://github.com/sulu-cmf/docs/blob/master/detail-specification/300-webspaces/DET-301-template-architecture.md#internal-structures
But in any case a message is better than getting an Exception and not knowing why. Its also good documentation, i.e. what does it mean when `$value !=== $node->getIdentifier()` ?