WIP: Convert vertical XModule to pure XBlock.

@cpennington  These are the changes we had to make to get VerticalModule running as a pure XBlock in the LMS - still raises in Studio. 

@nasthagiri @itsbenweeks FYI
I wonder whether `def runtime` above could just always return a `PureSystem`, so that a descriptor that had been bound as an xmodule would still call the xmodule get_child (and thus pass the binding to a student down the chain).