drag-and-drop ipynb to iomd converter

@hamilton has started looking at adding functionality that would allow drag-n-dropping a file on the iodide site pages, and which would then create an NB with a template showing how to load those files and start working with them.

once that functionality is built (particularly the server APIs), it would be an easy extension of that to allow people to DnD .ipynb files into the iodide site pages, parse the code and markdown out of the .ipynb, convert it to IOMD, and populate a new NB with with that iomd.

this would be a good first project once @hamilton gives the all clear that the prerequisites are built.

cc @willingc -- thanks for suggesting this the other day, great idea :-)