(WIP) Studio library view - Do not merge, for internal review

Turns out this `child_context['child_of_vertical'] = True` was not needed - it's only used to add some markup to VerticalModules used as "units" in a course. See https://github.com/edx/edx-platform/blob/5364508e/common/djangoapps/xmodule_modifiers.py#L182

I have removed it in the newer branch that supports randomization.