Move login links to components

We're trying to be consistent; in the past, we had the partials
"shared/admin_login_items", "layouts/notification_item" and
"devise/menu/login_items". Now we're moving all these partials to
components in the `Layout` namespace.

app/components/layout/admin_login_items_component.rb

@@ -0,0 +1,12 @@
+class Layout::AdminLoginItemsComponent < ApplicationComponent
+  attr_reader :user
+  delegate :show_admin_menu?, to: :helpers
+
+  def initialize(user)
+    @user = user
+  end
+
+  def render?
+    show_admin_menu?(user)
+  end
+end