Merge branch 'support-file-attachments-milestones' into 'master'

Add file attachment support in Milestone description

### What does this MR do?

This MR adds support for attaching files to Milestone descriptions.

### Why was this MR needed?

Dropzone support should be in every Markdown area. It seems that this one was overlooked.

### What are the relevant issue numbers?

Closes #1648

See merge request !712

CHANGELOG

@@ -5,6 +5,7 @@ v 7.12.0 (unreleased)
   - Fix Markdown preview not working in Edit Milestone page (Stan Hu)
   - Fix Zen Mode not closing with ESC key (Stan Hu)
   - Allow HipChat API version to be blank and default to v2 (Stan Hu)
+  - 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)
   - Remove Rack Attack monkey patches and bump to version 4.3.0 (Stan Hu)

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'