HTTP 404 for not found pages

Currently for SPA navigation the backend returns index.html for every URL (`rems.handler/not-found-handler`) and the frontend will determine whether the page exists and render a page-not-found message. By making the backend aware of the SPA routes (move secretary stuff from `rems/spa.cljs` to a `.cljc` file), the backend could return a HTTP 404 code and backend-generated error page for not found pages.