Add file attachment support in Milestone description

Closes #1648

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.12.0 (unreleased)
+  - Add file attachment support in Milestone description (Stan Hu)
   - Add web hook support for note events (Stan Hu)
   - Disable "New Issue" and "New Merge Request" buttons when features are disabled in project settings (Stan Hu)
   - Allow to configure location of the `.gitlab_shell_secret` file. (Jakub Jirutka)

app/assets/javascripts/dispatcher.js.coffee

@@ -27,6 +27,7 @@ class Dispatcher
         new Milestone()
       when 'projects:milestones:new', 'projects:milestones:edit'
         new ZenMode()
+        new DropzoneInput($('.milestone-form'))
       when 'projects:compare:show'
         new Diff()
       when 'projects:issues:new','projects:issues:edit'