Issues/464

This is branched from https://github.com/freedomofpress/securedrop/pull/463, and resolves #464.  This adds filters for the journalist interface, both the overview of sources and list of documents.  It also makes sure that any souces / submissions that are not visible will not be deleted/starred/unstarred.  Only visible items will be acted upon.  This preserves the graceful degradation of both `index` and `col`.
<pre>
  $("span.unread a").click(function(){
    sid = $(this).data('sid');
    $("span.unread[data-sid='" + sid + "']").remove();
  });
</pre>

I didn't want to assume the structure of the markup here, so I'm referencing it in two places (get the sid from the a, apply remove on span)