Enable multiple contacts for clients

Now, multiple contacts for a client can be synced as well.

IN always stores a new version of contacts when an invoice gets updated. So we do not store the contact ID on the Drupal side. Now we are up-to-date regarding contacts after updating a client. IN does not provide an API endpoint for contacts anyway, and that update does not cost any additional performance. They are always attached to a specific client.

But let's discuss.