Adding 'supercedes', 'supercededBy' to per-property page.
TODO: currently the entry e.g. for 'seller' in
http://schema.org/Offer won't list multiple properties that it
replaces. Fixing that (existing) bug will need a minor API change
since supercedes() only returns the first superceded property. The
only case of that is in the sdo-sellerbroker branch, so we'll come
to that afterwards.