Let $wgCategoryCollation take a class name as a value so that extensions

Let $wgCategoryCollation take a class name as a value so that extensions
can define new Collation classes.

(I plan to commit such an extension shortly)

Wasn't sure if it would be better to make an array mapping collation names => class names
instead. However, that seemed to be unneededly complicated so I went with
letting that variable take class names.