visually distinguish instructions from guided discussion section

common/lib/xmodule/xmodule/css/annotatable/display.scss

@@ -17,6 +17,8 @@ $body-font-size: em(14);
   border-radius: .5em;
   margin-bottom: .5em;
 
+  &.shaded { background-color: #EDEDED; }
+
   .annotatable-section-title {
     font-weight: bold;
     a { font-weight: normal; }

lms/templates/annotatable.html

@@ -6,7 +6,7 @@
     </div>
 
     % if instructions_html is not UNDEFINED and instructions_html is not None:
-    <div class="annotatable-section">
+    <div class="annotatable-section shaded">
         <div class="annotatable-section-title">
             Instructions
             <a class="annotatable-toggle annotatable-toggle-instructions expanded" href="javascript:void(0)">Collapse Instructions</a>