Measurements in user contributions should be able to get deleted again

[norbertschuler]

Describe the feature you'd like to see included in Plant-for-the-Planet App. Currently in the web client there is no possibility to delete measurements again and in the native apps the user can disable a measurement by disabling it with a switch, but the measurement is not deleted when updating the user contribution (payload still contains the measurement, so the backend keeps it).

Also the backend should support deleting of old measurements not being sent any more in the payload of an updating API call (delete them and send back "unlink" and "delete" objects in the response).

What are the use cases for this feature? Being able to delete wrong measurements.