[FEATURE] relational data in account/contact api

__Tasks:__

- [x] flatten relations like email
- [x] add main flag to email, fax, url, phone
- [x] adapt tests
- [x] add abstract contact class to avoid code duplications
- [ ] gather feedback for my changes 

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | [none]
| Doc           | none