Do variable substitution for detailed solutions

common/lib/capa/capa/capa_problem.py

@@ -265,7 +265,7 @@ class LoncapaProblem(object):
         # include solutions from <solution>...</solution> stanzas
         for entry in self.tree.xpath("//" + "|//".join(solution_types)):
             answer = etree.tostring(entry)
-            if answer: answer_map[entry.get('id')] = answer
+            if answer: answer_map[entry.get('id')] = contextualize_text(answer, self.context)
 
         log.debug('answer_map = %s' % answer_map)
         return answer_map