Revert "Merge pull request #256 from open-craft/rebase-20140926-merged-xblock-discussion-changes"

This reverts commit 9b079037d73bb12d85283e5a9004470f35f301ce, reversing
changes made to f5799fe261d9dcc6c10a8f69e45f0f4168e539f0.
Partially reapplied merge to enable discussion XBlock alongside discussion XModule