Put SequenceDescriptor back as a subclass of MakeModuleDescriptor. THe...

Put SequenceDescriptor back as a subclass of MakeModuleDescriptor. THe previous change to have it derive from EditorDescriptor was not safe as EditorDescriptor presumes that there is a edit box in the JS save flows. So we need to edit get_context() on MakoModuleDescriptor to also pass along the metadata to the Mako rendering templates