core&ui: fix pagination sort parameter (#1094)

packages/ui-default/templates/problem_main.html

@@ -8,7 +8,7 @@
         <div class="section__body no-padding">
           <form method="get" id="searchForm" action="{{ url('problem_main') }}">
             <div class="search">
-              <input name="q" type="text" value="{{ qs }}" placeholder="{{ _('Search') }}">
+              <input name="q" type="text" value="{{ q }}" placeholder="{{ _('Search') }}">
               {% include "partials/problem_stat.html" %}
               <select name="sort" class="search-sort">
                 {%- for sortType in ["default", "recent"] -%}