FIX: if the email template has multiple subjects, link to customize text page

app/assets/javascripts/admin/controllers/admin-customize-email-templates-edit.js.es6

@@ -4,6 +4,15 @@ import { bufferedProperty } from 'discourse/mixins/buffered-content';
 export default Ember.Controller.extend(bufferedProperty('emailTemplate'), {
   saved: false,
 
+  hasMultipleSubjects: function() {
+    const buffered = this.get('buffered');
+    if (buffered.getProperties('subject')['subject']) {
+      return false;
+    } else {
+      return buffered.getProperties('id')['id'];
+    }
+  }.property("buffered"),
+
   actions: {
     saveChanges() {
       const buffered = this.get('buffered');