Treat WikiLink/Category like PageProp/Category while serializing

* We've had bug reports on the VE/F page about new categories
  not serializing on a new line. Found that this is because
  VE temporarily went back to the old RDF annotations when we
  reverted our deploy last week.

* This patch makes us backward compatible till a new version of
  VE is deployed with the new RDF annotations for categories.

Change-Id: I2cc6b9bcb7bdc8c952ce8bec2ed123a49250253d