Add {{ get_string }} to docket page navigation within filter bar

The docket page navigation buttons within the filter bar loses any
parameters that are on the current page.  That means that it is possible
to loose sort order and other parameters.

cl/opinion_page/templates/includes/de_filter.html

@@ -84,7 +84,7 @@
         <div class="tight-input col-xs-6 hidden-sm col-sm-6 col-md-1 col-lg-2" >
           <div class="pull-right" >
             {% if docket_entries.has_previous %}
-              <a class="btn btn-default" href="?page={{ docket_entries.previous_page_number }}" rel="prev" >
+              <a class="btn btn-default" href="?{{ get_string }}page={{ docket_entries.previous_page_number }}" rel="prev" >
                 <i class="fa fa-caret-left" ></i><span class="hidden-md" >&nbsp;Prev.</span>
               </a>
             {% else %}
@@ -93,7 +93,7 @@
               </a>
             {% endif %}
             {% if docket_entries.has_next %}
-              <a class="btn btn-default" href="?page={{ docket_entries.next_page_number }}" rel="next" >
+              <a class="btn btn-default" href="?{{ get_string }}page={{ docket_entries.next_page_number }}" rel="next" >
                 <span class="hidden-md" >Next&nbsp;</span><i class="fa fa-caret-right"></i>
               </a>
             {% else %}