[FEATURE] n:n relation between accounts and contacts

__Tasks:__

- [x] add n:n relation between accounts-contacts
- [x] adapt REST API
- [ ] gather feedback for my changes 

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | [none]
| Doc           | none