core&ui: problem_main: support sort

packages/ui-default/templates/problem_main.html

@@ -10,6 +10,11 @@
             <div class="search">
               <input name="q" type="text" value="{{ qs }}" placeholder="{{ _('Search') }}">
               {% include "partials/problem_stat.html" %}
+              <select name="sort" class="search-sort">
+                {%- for sortType in ["default", "recent"] -%}
+                  <option value="{{ sortType }}" {{ "selected" if sort == sortType else "" }}>{{ _('sort::' + sortType) }}</option>
+                {%- endfor -%}
+              </select>
               <button type="submit">
                 <span class="icon icon-search"></span>
               </button>