Decode html entities before comparing strings in #ifeq: and #switch

This makes foo's equal to foo&#39;s. This is particularly important
since {{PAGENAME}} and friends html encode certain characters.

This change should be communicated to users before it hits wikis.
I don't think anyone is relying on the current behaviour, but
you never know.

Bug: 35628
Change-Id: I0035ac541d22491fcf15f55f2589aaab394ed0ed