Add trial banner support

app/views/layouts/header/_new.html.haml

-%header.navbar.navbar-gitlab.navbar-gitlab-new{ class: nav_header_class }
+- if current_user.admin? && license_message.present?
+  .alert.alert-dismissible.gitlab-ee-trial-banner.hidden.js-gitlab-ee-trial-banner{ role: 'alert', 'data-license-expiry' => current_license.expires_at }
+    %button.close{ type: 'button', 'data-dismiss' => 'alert', 'aria-label' => 'Dismiss banner' }
+      = icon('times', 'aria-hidden' => 'true')
+    %p
+      = license_message
+%header.navbar.navbar-gitlab.navbar-gitlab-new.js-navbar-gitlab{ class: nav_header_class }
   %a.sr-only.gl-accessibility{ href: "#content-body", tabindex: "1" } Skip to content
   .container-fluid
     .header-content