FLOE-228: Restructure repository to support manually building the EPUB

There are still some validation warnings related to unused files in the archive, but these will be resolved with other branches. We should also, at some point, work on some automation of the process, e.g. a Grunt script.
We may want to provide a more general process that is universal to all systems: i.e. "in physics-forces_and_motion/html directory, create a ZIP archive containing...". Perhaps we can keep the current lines 49-51 as an example.