Fix for XSS vulnerability in CategoryGallery

Improper escaping in CategoryGallery::getArticleSnippet allows for arbitrary HTML insertion. I'm not sure if I should code this in the ArticleService class itself, as there appears to be escaping present.

Example: When the page http://swtor.wikia.com/wiki/Videos was displayed within http://swtor.wikia.com/wiki/Category:Candidates_for_deletion , the <object> tag therein was rendered.