Make grouped milestone page public

Grouped milestone page now will skip authentication.

Solves #13403 which was a created in response to
https://gitlab.com/gitlab-com/www-gitlab-com/issues/554

CHANGELOG

@@ -33,6 +33,7 @@ v 8.6.0 (unreleased)
   - Fix bug where Bitbucket `closed` issues were imported as `opened` (Iuri de Silvio)
   - Don't show Issues/MRs from archived projects in Groups view
   - Fix wrong "iid of max iid" in Issuable sidebar for some merged MRs
+  - Make grouped milestone page public (Rodrigo Souto)
   - Increase the notes polling timeout over time (Roberto Dip)
   - Add shortcut to toggle markdown preview (Florent Baldino)
   - Show labels in dashboard and group milestone views

app/controllers/groups/milestones_controller.rb

 class Groups::MilestonesController < Groups::ApplicationController
   include GlobalMilestones
+  skip_before_action :authenticate_user!, only: [:show]
   before_action :projects
   before_action :milestones, only: [:index]
   before_action :milestone, only: [:show, :update]

spec/controllers/groups/milestones_controller_spec.rb

@@ -8,20 +8,40 @@ describe Groups::MilestonesController do
   let(:title) { '肯定不是中文的问题' }
   before do
-    sign_in(user)
-    group.add_owner(user)
-    project.team << [user, :master]
     controller.instance_variable_set(:@group, group)
   end
-  describe "#create" do
-    it "should create group milestone with Chinese title" do
-      post :create,
-           group_id: group.id,
-           milestone: { project_ids: [project.id, project2.id], title: title }
-      expect(response).to redirect_to(group_milestone_path(group, title.to_slug.to_s, title: title))
-      expect(Milestone.where(title: title).count).to eq(2)
+  context 'unauthenticated user' do
+    describe "#show" do
+      it "should display milestone" do
+        create(:milestone, title: title, project: project)
+        get :show,
+          group_id: group.id,
+          id: title,
+          title: title
+
+        expect(response).to be_success
+      end
+    end
+  end
+
+  context 'project owner' do
+    before do
+      sign_in(user)
+      group.add_owner(user)
+      project.team << [user, :master]
+    end
+
+    describe "#create" do
+      it "should create group milestone with Chinese title" do
+        post :create,
+             group_id: group.id,
+             milestone: { project_ids: [project.id, project2.id], title: title }
+
+        expect(response).to redirect_to(group_milestone_path(group, title.to_slug.to_s, title: title))
+        expect(Milestone.where(title: title).count).to eq(2)
+      end
     end
   end
 end