Use github API to upload harvest results

[Zalfsten]

Currently the basic middleware checks out the github repo 'fairagro_repo', makes local changes and pushes them back to github. The advantage is that this works with every git server, not just with github. But the requirement for local storage violates the 12-factor principles for cloud/docker applications. Also the management of the required ssh key is kind of awkward. A solution would be to perform a "remote commit" using the github API. This is what we also intend to do for the advanced middleware with the gitlab API.