Add trackEvent when accessing the Register tab

app/views/devise/shared/_tabs_normal.html.haml

@@ -3,4 +3,4 @@
     %a.nav-link.qa-sign-in-tab.active{ href: '#login-pane', data: { toggle: 'tab' }, role: 'tab' } Sign in
   - if allow_signup?
     %li.nav-item{ role: 'presentation' }
-      %a.nav-link.qa-register-tab{ href: '#register-pane', data: { toggle: 'tab' }, role: 'tab' } Register
+      %a.nav-link.qa-register-tab{ href: '#register-pane', data: { track_label: 'sign_in_register', track_property: 'sign_in', track_event: 'click_button', track_value: 'register', toggle: 'tab' }, role: 'tab' } Register

ee/app/assets/javascripts/pages/sessions/index.js

+import '~/pages/sessions/index';
+import initSignInRegisterTracking from './sign_in_register_tracking';
+
+document.addEventListener('DOMContentLoaded', initSignInRegisterTracking);

ee/app/assets/javascripts/pages/sessions/sign_in_register_tracking.js

+import Stats from 'ee/stats';
+
+export default () => {
+  Stats.bindTrackableContainer('#signin-container');
+};