Fix for related links, also fix for scenario where filter query string values are blank

Related links: will no longer coerce a list within a list into a string (which is what was happening for related_links)

Filter query strings:  sheer will now only add filters to the ES query if they have values.  For example, "&filter_range_date_gt=" will now simply make no change to what is filtered.  Same goes for "&filter_category=".  Previously, "&filter_range_date_gt=" was blowing up the app, while "&filter_category=" would try to match the category of an empty string, returning no results.