Separate fromhidden/tohidden indexes in database upgrade

Any users who ran the upgrade in the previous commit should delete the
fromhidden index and replace with separate indexes on the fromhidden
and tohidden fields.

qa-include/qa-db-install.php

@@ -1419,7 +1419,7 @@
 						else {
 							qa_db_upgrade_query('ALTER TABLE ^messages ADD COLUMN fromhidden '.$definitions['messages']['fromhidden'].' AFTER touserid');
 							qa_db_upgrade_query('ALTER TABLE ^messages ADD COLUMN tohidden '.$definitions['messages']['tohidden'].' AFTER fromhidden');
-							qa_db_upgrade_query('ALTER TABLE ^messages ADD KEY fromhidden (fromhidden, tohidden)');
+							qa_db_upgrade_query('ALTER TABLE ^messages ADD KEY fromhidden (fromhidden), ADD KEY tohidden (tohidden)');
 
 							qa_db_upgrade_query($locktablesquery);
 						}