* Add ve.init.mw.LinkCache to track page existence and
transparently query it
* Populate it with initial data from the parser cache
if available, obtained in the VE API module
* Use linkCache data in link annotation rendering
This doesn't yet integrate the LinkCache with other
components like the link inspector. That should be
done so we can deduplicate the existence checks.
Additionally, we should generalize LinkCache and use
it for the category existence/status checks as well.
Bug: 37901
Change-Id: I9fd43e8c3864dd375cf6dadfdeedd05e4fe9cf3b
