OutputPage: Set wgPageContentLanguage to pageViewLang instead of pageLang

OutputPage: Set wgPageContentLanguage to pageViewLang instead of pageLang

Follows-up 80a090ea7 (r104483).

I'm assuming the difference is that pageLanguage will be the canonical variant
for the page (as authored in wikitext, presumably the wiki's default), whereas
pageViewLanguage will be the variant of the pageLanguage based on the current
user-interface language (if and only if the user interface is related).

Viewing zh by default has:
* userLanguage: zh
* pageLanguage: zh
* pageViewLanguage: zh

Viewing zh-cn by default has:
* userLanguage: zh
* pageLanguage: zh
* pageViewLanguage: zh-cn

Viewing zh-cn with uselang=en has:
* userLanguage: en
* pageLanguage: zh
* pageViewLanguage: zh-cn

Viewing zh-cn with uselang=zh-tw has:
* userLanguage: zh-tw
* pageLanguage: zh
* pageViewLanguage: zh-cn

The main use case behind wgPageContentLanguage in 80a090ea7 was tablesorter
which should match the language and variant of the rendered wikipage content.

The parser output and div#mw-content-text element use pageViewLanguage, so
let's set wgPageContentLanguage to that as well.

Change-Id: Ibf399dfe5ca60fb55e7acd75a5c0238d8b903807