main: Serve static files from /static path

Static files served with FileServer have a low priority : all other possible routes
a this root will be checked first, before they get returned.
As we have a lot of routes at the root, they will be hit a lot before we get
to the static files.

Move the static files to a dedicated path to reduce those internal redirections,
update the paths in the HTML and add one for the favicon so it continues to work.

src/main.rs

@@ -35,7 +35,7 @@ async fn index(mut db: Connection<Db>) -> Redirect {
 #[launch]
 fn rocket() -> _ {
     rocket::build()
-        .mount("/", FileServer::from(relative!("static_files")))
+        .mount("/static/", FileServer::from(relative!("static_files")))
         .attach(auth::stage())
         .attach(week::stage())
         .attach(truth::stage())