Add link to substance class view in subadd

[dgets]

I'm actually not sure that this would be the best place for this right now, maybe it should be at the top navigation menu... I think for now it'd be better off in the subadd bottom navigation bar, though, so we'll just go with that. There just needs to be a link to the substance classes view there, or else the link to 'add substance class' needs to be renamed to 'Substance Classes' or something of the sort.