Making category sort keys work for blog posts

As of now, categories are using hardcoded sorting, ordered by username and blog title. This change allows custom sort keys added using {{DEFAULTSORT:sortkey}} or [[Category:Categoryname|sortkey]].
Update CategoryViewer::addPage hook handler in Answers

The changes made to the CategoryViewer::addPage hook in VOLDEV-29 necessitate an update of its handler answerAddCategoryPage in Answers.php so that it can process the extra parameter (string $humanSortkey).
Add sortkey parameter to the other hook call
Reorder parameters
Merge pull request #2426 from WikiaVolDev/VOLDEV-29

VOLDEV-29