Update GeSHi integration

Idf4ad439 deprecated SyntaxHighlight_GeSHi::buildHeadItem in favor of
ResourceLoader. We may as well follow along.

To avoid breaking things if people don't update their
SyntaxHighlight_GeSHi in sync with Scribunto, test for the RL class used
in GeSHi's ResourceLoader integration and continue using buildHeadItem
if it's not found.

Change-Id: Id93530dce7c8c5a70a98fca58844adf9b5c6a23f
Merge "Update GeSHi integration"