About attribute values should have "#" prefixed everywhere

* While debugging reports of dirty diffs on some pages (en:Bleak House
  specifically), it took me a while to notice that some mw:Transclusion
  had about="mwt5" style ids (introduced during template expansion reuse)
  whereas all other about ids had about="#mwt5" (note the # char) style ids.

  While this by itself shouldn't cause dirty diffs since DOM-diff ignores
  about ids, this could potentially introduce introduce other bugs elsewhere
  if we start using/comparing about ids.

  Fixed all uses of "#" + env.newObjectId() with env.newAboutId and let
  env prefix the "#" key.

Change-Id: I74d50ae155f5d24af95c07da15b14eb990cf2891