Added {{REVISIONSIZE}} variable to get the current size of a revision.

This avoids giving the own page name to {{PAGESIZE:}} and allows
PAGESIZE on subst: to give the size before save.

Viewing a oldid page will give the corresponding size to that revision.
(Like REVISIONUSER or REVISIONTIMESTAMP)

This partially reverts Idfac13de37d05317f65e4131534543e66cf74873
because there is no need to let PAGESIZE handle extra, when the given
title is the own page.

Change-Id: If9e608d54904c21ac6b095e91ff6e0a15da0fb4c