Feature/account-contacts

depends on pr: https://github.com/sulu-cmf/sulu/pull/102

__Tasks__
- [x] add contacts list to account details
- [x] adapt Response in contact and account controller

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- [x] gather feedback for my changes
- [x] test coverage


| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none