Use new ContentGetParserOutput hook.

Using ContentGetParserOutput instead of ShowRawCssJs allows highliting to be applied
for other kinds of scripts as well (e.g. Lua). It also allows more special case code
for CSS and JS to be phased out.

NOTE: this requires Ibfb2cbef to be merged in core!

Change-Id: Ie260c22680ec9a31e505c685d70e17efe8a7bf44