enforce display name in CAPA problems

FEDX-198

lms/templates/problem.html

@@ -1,4 +1,8 @@
-<%! from django.utils.translation import ugettext as _ %>
+<%page expression_filter="h"/>
+<%!
+from django.utils.translation import ugettext as _
+from openedx.core.djangolib.markup import HTML
+%>
 
 <%namespace name='static' file='static_content.html'/>
 <h3 class="hd hd-2 problem-header">
@@ -9,7 +13,7 @@
 
 <div class="problem">
     <div aria-live="polite">
-        ${ problem['html'] }
+        ${ HTML(problem['html']) }
     </div>
   <div class="action">
     <input type="hidden" name="problem_id" value="${ problem['name'] }" />