queue: Add MySQL index hint

This adds the advanced option to the queue sort configuration. An index can be
specified to be used for the sorting operation. In some cases, the MySQL query
optimizer cannot select the most efficient index to use when dealing with large
querysets and sorting. This feature, if enabled, allows an administrator to specify
an index which MySQL should use when using the sort.

To use the feature, an `extra` column must be added to the `%queue_sort` table to
receive the index name.