core: support search by difficulty

packages/ui-default/templates/problem_main.html

@@ -2,9 +2,6 @@
 {% import "components/problem.html" as problem with context %}
 {% extends "layout/basic.html" %}
 {% block content %}
-  {{ set(UiContext, {
-      currentCategory: category
-  }) }}
   <div class="row">
     <div class="medium-9 columns">
       <div class="section display-mode">