add (correct) support for logged in users to sba and physics exams

sbas/templates/sbas/exam_start.html

@@ -4,6 +4,9 @@
 
     <h2>Start exam: {{exam.name}}</h2>
 
+    {% if request.user.is_authenticated and valid_user %}
+    <a href="{% url 'sbas:exam_take_user' pk=exam.pk sk=0 %}">Start Exam</a>
+    {% else %}
     Enter your CID and passcode in the below boxes.<br />
     <p><input id="cid-box" type="text" value="Candidate ID"></p>
     <p><input id="passcode-box" type="text" value="Passcode"></p>
@@ -44,4 +47,5 @@
             });
         });
     </script>
+    {% endif %}
 {% endblock %}