Web server error page i18n

[rdmark]

We have a 503 error page that is displayed by NGINX while rascsi-web is starting up. Since this is loaded outside of the flask app context, we need a separate approach to i18n.

Some discussion in https://stackoverflow.com/questions/43387522/set-custom-404-error-page-on-nginx-based-on-locale-through-url-parameter