Apply $wgMaxArticleSize more exactly

$wgMaxArticleSize is defined as "maximum article size in kilobytes",
however the way it was being used in WikiImporter and EditPage was
actually allowing 1023 bytes more than the limit. Other code using the
variable was limiting it to the specified value.

Change-Id: I85e4d2146643c5ac65f27cf464a51b28d68440b0