Fix diff image view modes (2-up, swipe, onion skin) not working

Closes #13856

CHANGELOG

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.6.0 (unreleased)
   - Add confidential issues
   - Bump gitlab_git to 9.0.3 (Stan Hu)
+  - Fix diff image view modes (2-up, swipe, onion skin) not working (Stan Hu)
   - Support Golang subpackage fetching (Stan Hu)
   - Bump Capybara gem to 2.6.2 (Stan Hu)
   - New branch button appears on issues where applicable

app/assets/javascripts/application.js.coffee

@@ -7,6 +7,7 @@
 #= require jquery
 #= require jquery-ui/autocomplete
 #= require jquery-ui/datepicker
+#= require jquery-ui/draggable
 #= require jquery-ui/effect-highlight
 #= require jquery-ui/sortable
 #= require jquery_ujs