[FEATURE] Added navigation context in structure

Added navigation context in structure:

__Tasks:__

- [x] test coverage
- [ ] gather feedback for my changes
- [x] submit changes to the documentation
- [x] UI (https://github.com/sulu-cmf/SuluContentBundle/pull/186)
- [x] Tests ContentMapper Interface (https://github.com/sulu-cmf/SuluTestBundle/pull/17)
- [x] Bugfix husky (https://github.com/massiveart/husky/pull/343)
- [x] Add it to navigation mapper (https://github.com/sulu-cmf/SuluWebsiteBundle/pull/34)

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | no => SuluTestBundle
| Fixed tickets | https://github.com/sulu-cmf/sulu-standard/issues/121
| BC Breaks     | none
| Doc           | https://github.com/sulu-cmf/docs/commit/a3770dccc140095e2ec806441d47b974564ec071
@wachterjohannes Should here really be `true` passed? `NavContexts` are a string array.