Only allow fields from isFieldSortable to be sorted on.

Prevents people from doing stupid things like:
https://commons.wikimedia.org/w/index.php?title=Special%3AListFiles&limit=50&user=bawolff&sort=img_description&asc=&desc=1
Which don't have indexes.

Change-Id: I7aeb89032e89283dd7dfe5d7bf12872203ca5520