New variable: new_pst which provides a PSTed version of new_wikitext

It's observed that some vandals use {{subst:void}} to trick the system
and other users and this new variable may help.

It would be nice to be possible to diff this variable against
old_wikitext, but it needs some performance consideration.

Change-Id: I4c4c4e806663363b40936363d659c42016e18cf6
Merge "New variable: new_pst which provides a PSTed version of new_wikitext"