Add basic auth

For now this uses the SUPPORT_USERNAME/PASSWORD env vars.

app/controllers/application_controller.rb

@@ -1,3 +1,18 @@
 class ApplicationController < ActionController::Base
   default_form_builder(GOVUKDesignSystemFormBuilder::FormBuilder)
+
+  before_action :authenticate
+
+  def authenticate
+    valid_credentials = [
+      {
+        username: ENV.fetch("SUPPORT_USERNAME", "support"),
+        password: ENV.fetch("SUPPORT_PASSWORD", "support"),
+      },
+    ]
+
+    authenticate_or_request_with_http_basic do |username, password|
+      valid_credentials.include?({ username:, password: })
+    end
+  end
 end