Add proper file api endpoint for cxx

This merge request implements a new endpoint for the upload and download of the "code" for binary algorithms. Currently C++ algorithms.

The current method used by the website is not the most cleanest implementation and should be replaced by the endpoint in the long run.

This will also allow to properly push and pull from beat/beat.cmdline>

As a drive by fix, it also add the missing writing_serializer_class variable to the RetrieveUpdateDestroyExperimentView.

Part of beat.cmdline#73 (closed)