Hide 'timedout' flashes from Devise

Devise uses `flash[:timedout]` for recording when a session is timed out and
this is automatically displayed to the user. Adding `timedout` to the list
of messages to hide in `app/views/layouts/_flashes.html.haml` hides this.

For reference: heartcombo/devise#1777

app/views/layouts/_flashes.html.haml

@@ -1,3 +1,3 @@
 - flash.each do |key, value|
-  - unless %w{hidden ga}.include? key
+  - unless %w{hidden ga timedout}.include? key
     = govuk_notification_banner(key.capitalize, value, class: "govuk-notification-banner--#{key}")