Rename the iwl_prefix_from_title index (again) to iwl_prefix_title_from and change...

Rename the iwl_prefix_from_title index (again) to iwl_prefix_title_from and change the field order accordingly. Fixed r66892 which inserted iwl_from into the index (which was a good thing) but put it in the wrong place. I went out of my way to make sure the index isn't dropped and recreated needlessly, but since I don't know how to do the drop-index-if-exists thing in the Postgres updater, I left that out. The Postgres updater will now create the new index without dropping any old incarnations if present. I did create the patch files with the DROP INDEX statements to make it easier to add this behavior. Also tweak ApiQueryIWBacklinks to use this index properly.