core: add maintainer field for doc (#101)

packages/ui-default/templates/partials/problem_sidebar_homework.html

@@ -28,7 +28,7 @@
         <span class="icon icon-send"></span> {{ _('Open in Problem Set') }}
       </a></li>
     {% endif %}
-    {% if pdoc.owner == handler.user._id or handler.user.hasPerm(perm.PERM_EDIT_PROBLEM) %}
+    {% if handler.user.own(pdoc) or handler.user.hasPerm(perm.PERM_EDIT_PROBLEM) %}
       <li class="menu__seperator"></li>
       <li class="menu__item"><a class="menu__link{% if page_name == 'problem_edit' %} active{% endif %}" href="{{ url('problem_edit', pid=pdoc.pid|default(pdoc.docId)) }}">
         <span class="icon icon-edit"></span> {{ _('Edit') }}