[FEATURE] internal links content type

implemented internal links content type:

__Tasks:__

- [x] test coverage
- [x] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | no => depending PR https://github.com/sulu-cmf/SuluContentBundle/pull/137
| Fixed tickets | none
| Doc           | none