Fix for the sorting of Works not sorting correctly

In order to sort these fields there needs to be a non-multivalued field in Solr for each. For some reason there was just a multivalued field getting indexed.

The old code tries to generate two solr fields (one multivalued, one not), but that code didn't work correctly. I'm not sure if that's a Solrizer bug or a configuration bug in Curate. In any case, there isn't a need to have the multivalued fields for date uploaded and date modified, so we just use the :stored_sortable Solrizer symbol. A reindex needs to run for this change (ActiveFedora::Base.reindex_everything is useful).