[FEATURE] flat representation of account / contact main-relations

__Tasks:__
- [x] create entity for address - contact / account relation
- [x] handle processing of address relations
- [x] create fields: ( main-email, main-phone, main-fax, main-url ) in account and contact
- [ ] gather feedback for my changes 

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | [none]
| Doc           | none
yes, because otherwise contact will not have the relation when returning the entity directly (without fetching it from db)