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
Do you remember what broke before you made this change? Was it that when you had an `XModuleDescriptor` with a `DescriptorSystem`, it used the `DescriptorSystem.get_child` even when the `XModuleDescriptor` had had `bind_for_student` had been called?