(BugId:100980) - Fix losing categories inside of templates on save.

https://wikia.fogbugz.com/default.asp?100980

Adds a parser preprocess call so that templates will be expanded inside of the article wikitext when new categories are added. I'm afraid this might be slow on huge articles, but it probably doesn't matter because it's an AJAX request. Nevertheless, open to better ways of scraping for all the categories on a page.