Don't wipe out CKEDITOR instances if they already exist

This fixes an issue where HTML custom field inputs could
not be updated via inline edit on the ticket display page
on the first try, but the value could be updated on a
second Save.

share/static/js/util.js

@@ -368,7 +368,7 @@ function textToHTML(value) {
 };
 
 function ReplaceAllTextareas(elt) {
-    CKEDITOR = { "instances": {} };
+    window.CKEDITOR ||= { "instances": {} };
 
     elt ||= document;
     // replace all content and signature message boxes