[FEATURE] edit Contacts in Account-List

__Tasks:__

- [x] bugfix: activate save button after birthdayfield was initialized
- [x] REST Api for AccountContact
- [x] Repository Class for AccountContact
- [x] add and remove contacts in account-contact-list
- [ ] gather feedback for my changes 

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | [none]
| Doc           | none
@turbo-ele not needed?