Accept either a list of possible values, or a string as a value for

comparison of correctness in multiple choice.

Multiple choice code is scattered and sometimes sends a list of
choices for the value, and sometimes a single string. We used to use
"in" which scarily handled both cases (list or substring search),
but that caused a bug when you had two choices like choice_1 and
choice10. Moving to == caused us to break when lists were sent to
us. So this ugly code is extra paranoid and checks both possibilities.

This really needs a better cleanup.

common/lib/capa/capa/templates/choicegroup.html

@@ -17,7 +17,7 @@
 
         % for choice_id, choice_description in choices:
         <label for="input_${id}_${choice_id}"
-            % if input_type == 'radio' and choice_id == value:
+            % if input_type == 'radio' and ( (isinstance(value, basestring) and (choice_id == value))  or  (not isinstance(value, basestring) and choice_id in value) ):
             <% 
                 if status == 'correct':
                     correctness = 'correct'
@@ -30,9 +30,9 @@
             class="choicegroup_${correctness}"
             % endif
             % endif
-            >  
+            >
             <input type="${input_type}" name="input_${id}${name_array_suffix}" id="input_${id}_${choice_id}" value="${choice_id}"
-            % if input_type == 'radio' and choice_id == value:
+            % if input_type == 'radio' and ( (isinstance(value, basestring) and (choice_id == value))  or  (not isinstance(value, basestring) and choice_id in value) ):
             checked="true" 
             % elif input_type != 'radio' and choice_id in value:
             checked="true"