Rocket: Use fairings for routes of sub-files

Instead of adding all routes manually in the Launch function for Rocket,
implement fairings for all the different files that adds the routing to
the rocket in a self-contained manner.

src/week.rs

@@ -210,3 +210,9 @@ pub async fn create_week(week: u8, mut db: Connection<Db>, cookies: &CookieJar<'
             }
     }
 }
+
+pub fn stage() -> AdHoc {
+    AdHoc::on_ignite("Week stage", |rocket| async {
+        rocket.mount("/", routes![week, create_week, update_week, set_last_week])
+    })
+}