合并dev,修复冲突，修改比赛列表中的小bug

template/src/oj/problem/problem.html

@@ -9,9 +9,6 @@
             <li role="presentation">
                 <a href="/problem/{{ problem.id }}/submissions/">我的提交</a>
             </li>
-            <li role="presentation">
-                <a href="/problems/">返回</a>
-            </li>
         </ul>
         {% include "oj/problem/_problem_header.html" %}
 

template/src/oj/problem/problem_list.html

@@ -21,16 +21,16 @@
                             <th></th>
                             <th>#</th>
                             <th>题目</th>
-                            <th><a href="/problems/?order_by=difficulty">难度</a></th>
-                            <th><a href="/problems/?order_by=acceptance">通过率</a></th>
+                            <th><a href="/problems/?order_by={{ difficulty_order }}">难度</a></th>
+                            <th>通过率</th>
                         </tr>
                         </thead>
                         <tbody>
                         {% for item in problems %}
                             <tr>
                                 <th><span class="glyphicon glyphicon-ok ac-flag"></span></th>
-                                <th scope="row"><a href="/problem/{{ item.id }}/" target="_blank">{{ item.id }}</a></th>
-                                <td><a href="/problem/{{ item.id }}/" target="_blank">{{ item.title }}</a></td>
+                                <th scope="row"><a href="/problem/{{ item.id }}/">{{ item.id }}</a></th>
+                                <td><a href="/problem/{{ item.id }}/">{{ item.title }}</a></td>
                                 <td>{{ item.difficulty }}</td>
                                 <td>{{ item|accepted_radio }}</td>
                             </tr>