allow briefcase to upload xforms and submissions

We use pyxform to try and deduce the json form from the xml, not all cases are handled. The branch https://github.com/XLSForm/pyxform/tree/model_instance_maybe_a_list has some cases handled. We will need access to the xlsform/xform of those which are still failing after this has been merged and deployed in order to troubleshoot and provide updates to the branch. The pyxform branch is up to date with master, I will be merging it with pyxform master after some code refactor not sure when.