Properly use django template filters in teachers_and_subjects_table

aleksis/apps/alsijil/templates/alsijil/partials/register_teachers_and_subjects_table.html

 <table id="lessons">
   <thead>
   <tr>
-    {% if len(groups) > 1 %}
+    {% if groups|length > 1 %}
       <th>{% trans 'Group' %}</th>
     {% endif %}
     <th>{% trans 'Subject' %}</th>
@@ -14,7 +14,7 @@
   {% for group in groups %}
     {% for course in group.courses.all %}
       <tr>
-        {% if len(groups) > 1 %}
+        {% if groups|length > 1 %}
           <td>{{ group.name }}</td>
         {% endif %}
         <td>{{ course.subject.name }}</td>