Display Merge Requests <-> Milestone relations

Show the relation between Merge Requests and Mileston in different
places:
- Merge Request page
- Milestone list page
- Milestone details page

app/controllers/milestones_controller.rb

@@ -32,6 +32,7 @@ class MilestonesController < ProjectResourceController
   def show
     @issues = @milestone.issues
     @users = @milestone.participants
+    @merge_requests = @milestone.merge_requests
 
     respond_to do |format|
       format.html