fix success response handling of sending digest preview email

app/assets/javascripts/admin/controllers/admin-email-preview-digest.js.es6

@@ -12,6 +12,7 @@ export default Ember.Controller.extend({
       const model = this.get('model');
 
       this.set('loading', true);
+      this.set('sentEmail', false);
       EmailPreview.findDigest(this.get('lastSeen'), this.get('username')).then(email => {
         model.setProperties(email.getProperties('html_content', 'text_content'));
         this.set('loading', false);

app/controllers/admin/email_controller.rb

@@ -60,7 +60,7 @@ class Admin::EmailController < Admin::AdminController
       message.to = params[:email]
       begin
         Email::Sender.new(message, :digest).send
-        render nothing: true
+        render json: success_json
       rescue => e
         render json: {errors: [e.message]}, status: 422
       end