Added ability to add and remove the forked from/to relatioinship
between existing repos.

Added a helper method to check if required parameters are given in an API call. Can be used
to return a `400 Bad Request` return code if a required attribute is missing.
Code clean up and fixed tests.

Extracted a method for 400 error (Bad request) and adjusted code accordingly. The name of
the missing attribute is used to show which one was missing from the request. It is used to
give an appropriate message in the json response.