Actually pass oldid parameter when serializing

Apparently Parsoid has always required this, and we've never sent it,
yet somehow the code in production works. This may well be the cause
of some of the selser issues we saw after the deployment attempt in
January.

Made oldid a required parameter in the API module, and default it to 0.
When we get 0, we translate that to the empty string for Parsoid's
benefit. We also need to explicitly get wgCurRevisionId in
ViewPageTarget, and that's also 0 on new pages.

Change-Id: I3a55025246014cd74e15d6d5b6c4ede7b823e5df
Merge "Actually pass oldid parameter when serializing"