Make chem preview more robust

- give the preview div an id, use that to look it up instead of dancing around in the DOM

common/lib/capa/capa/templates/chemicalequationinput.html

@@ -29,10 +29,8 @@
         % endif
       </p>
 
-      <div class="equation">
-        
+      <div id="input_${id}_preview" class="equation">
       </div>
-
       
   <p id="answer_${id}" class="answer"></p>
 

common/static/js/capa/chemical_equation_preview.js

 (function () {
     update = function() {
-        preview_div = $(this).siblings('div.equation');
-
         function create_handler(saved_div) {
             return (function(response) {
                 if (response.error) {
@@ -12,6 +10,9 @@
             });
         }
 
+        prev_id = "#" + this.id + "_preview";
+        preview_div = $(prev_id)
+
         $.get("/preview/chemcalc/", {"formula" : this.value}, create_handler(preview_div));
     }