Display flash notice about Geo node on all admin screens

app/controllers/admin/application_controller.rb

@@ -2,10 +2,18 @@
 #
 # Automatically sets the layout and ensures an administrator is logged in
 class Admin::ApplicationController < ApplicationController
+  include ActionView::Helpers::UrlHelper
   before_action :authenticate_admin!
+  before_action :display_geo_information
   layout 'admin'
 
   def authenticate_admin!
     render_404 unless current_user.is_admin?
   end
+
+  def display_geo_information
+    return unless Gitlab::Geo.secondary?
+    primary_node = link_to('primary node', Gitlab::Geo.primary_node.url)
+    flash.now[:notice] = "You are in a Geo secondary node (read-only). To make any change you must visit the #{primary_node}.".html_safe
+  end
 end