[Api2] Refactor contact endpoint

[rubenhorn]

see /contact in docs/api2.yaml

• [x] Successful response matches API spec
• [x] Error response matches API spec
• [x] Request is validated according to API spec (nonce refers to feature #58 and should not be a required field for now, but it should be allowed)
• [ ] Header X-CollActionAPI-Version is optional in request and loaded, but not further processed (maybe leave todo in the code)