add a `cancelled` status for callbacks - eHealthAfrica/sl-ebola-call-…

[danse]

Okay done. This is the fixed version of #85. I think that tests should be more strict and fail when there is an object which is not in the schema, anyway it is probably not worth working on this since the data models seem not often used recently

[emig]

@danse LGTM would you add the changes to the CHANGELOG?

[danse]

cool, i did not add anything because the current changelog still has callbacks unreleased, so this would not represent a change. if you want i can add a line anyway