Add browser and platform flags

- Adds CSS classes to represent browser & platform
- Adds JS flags to `gl` object to represent
browser & platform

app/views/layouts/devise.html.haml

 !!! 5
 %html.devise-layout-html{ class: system_message_class }
   = render "layouts/head"
-  %body.ui-indigo.login-page.application.navless{ data: { page: body_data_page, qa_selector: 'login_page' } }
+  %body.ui-indigo.login-page.application.navless{ class: "#{client_class_list}", data: { page: body_data_page, qa_selector: 'login_page' } }
     = header_message
+    = render "layouts/init_client_detection_flags"
     .page-wrap
       = render "layouts/header/empty"
       .login-page-broadcast

app/views/layouts/devise_empty.html.haml

 !!! 5
 %html{ lang: "en", class: system_message_class }
   = render "layouts/head"
-  %body.ui-indigo.login-page.application.navless
+  %body.ui-indigo.login-page.application.navless{ class: "#{client_class_list}" }
     = header_message
+    = render "layouts/init_client_detection_flags"
     = render "layouts/header/empty"
     = render "layouts/broadcast"
     .container.navless-container

spec/features/users/login_spec.rb

@@ -624,6 +624,14 @@ describe 'Login' do
     end
   end
 
+  describe 'Client helper classes and flags' do
+    it 'adds client browser and platform classes to page body' do
+      visit root_path
+      expect(find('body')[:class]).to include('gl-browser-generic')
+      expect(find('body')[:class]).to include('gl-platform-other')
+    end
+  end
+
   context 'when terms are enforced' do
     let(:user) { create(:user) }