Skylight instrumentation

This allows individual installations to optionally collect performance
data using [Skylight for Open Source](https://www.skylight.io/oss).

spec/controllers/application_controller_spec.rb

@@ -30,7 +30,45 @@ describe ApplicationController, type: :controller do
           expect(controller.after_sign_in_path_for(user)).to eq conferences_path
         end
       end
-
+    end
+
+  end
+
+end
+
+describe ApplicationController, type: :request do
+  let(:conference) { create(:conference) }
+
+  describe 'Skylight link' do
+
+    around do |example|
+      original_value = ENV['SKYLIGHT_PUBLIC_DASHBOARD_URL']
+      example.run
+      ENV['SKYLIGHT_PUBLIC_DASHBOARD_URL'] = original_value
+    end
+
+    context 'when SKYLIGHT_PUBLIC_DASHBOARD_URL is set' do
+      before do
+        ENV['SKYLIGHT_PUBLIC_DASHBOARD_URL'] = 'https://oss.skylight.io/app/applications/my-osem'
+      end
+
+      it 'should include a link to view performance data' do
+        get '/'
+        expect(response.body).to match(/Performance data/i)
+        expect(response.body).to include('https://oss.skylight.io/app/applications/my-osem')
+      end
+    end
+
+    context 'when SKYLIGHT_PUBLIC_DASHBOARD_URL is not set' do
+      before do
+        ENV['SKYLIGHT_PUBLIC_DASHBOARD_URL'] = nil
+      end
+
+      it 'should not include a link to view performance data' do
+        get '/'
+        expect(response.body).to_not match(/performance data/i)
+        expect(response.body).to_not match(/skylight/i)
+      end
     end
 
   end