Add group milestones routes and controller.

app/controllers/groups/milestones_controller.rb

+class Groups::MilestonesController < ApplicationController
+  layout 'group'
+
+  def index
+    @group = Group.find_by(path: params[:group_id])
+  end
+end

app/views/layouts/nav/_group.html.haml

@@ -12,6 +12,11 @@
       Merge Requests
       - if current_user
         %span.count= current_user.cared_merge_requests.opened.of_group(@group).count
+  = nav_link(path: 'groups#milestones') do
+    = link_to merge_requests_group_path(@group) do
+      Milestones
+      - if current_user
+        %span.count= current_user.cared_merge_requests.opened.of_group(@group).count
   = nav_link(path: 'groups#members') do
     = link_to "Members", members_group_path(@group)
 

config/routes.rb

@@ -151,8 +151,10 @@ Gitlab::Application.routes.draw do
     end
 
     resources :users_groups, only: [:create, :update, :destroy]
+
     scope module: :groups do
       resource :avatar, only: [:destroy]
+      resources :milestones
     end
   end