Article: Add RL modules for the ParserOutput when showing CSS/JS page

It previously added only the page HTML, which meant that ResourceLoader
modules associated with the page and other metadata was lost. This
affected the new way of doing things in SyntaxHighlight (Idf4ad439).

We still do not show categories, language links and other things
on the pages (although internally they behave just like in wikitext)

Also needs a fix in DifferenceEngine because our code sucks so much.

Bug: 64859
Change-Id: I5867985693b27ccc9195fddde8b922e67463e836
Merge "Article: Add RL modules for the ParserOutput when showing CSS/JS page"