Remove invalid context and fixed content that depends on Parent now that it's actually showing.

This one is a little different than the other "invalid context" PRs. It removes the 'node_add_form' context because that is also breaking Preview and it actually doesn't even work. However, after removing it, all the Panes that depended on the Parent context start actually appearing! :-)

So, this also rearranges those Panes so that the UX of editing a Discussion Post is good. It looks like the intention is simply to show the parent while you're editing the child, so that's what it does. Without these changes it also shows all the parent replies and a repeat of the parent body -- which is pretty messy and weird.