Add very rudimentary frontend crate

I accidentally pushed the dependency in main.rs :(

crates/frontend/src/lib.rs

@@ -0,0 +1,10 @@
+use actix_web::{
+    web::{self, Data},
+    HttpResponse,
+};
+use rustical_store::calendar::CalendarStore;
+use tokio::sync::RwLock;
+
+pub fn configure_frontend<C: CalendarStore>(cfg: &mut web::ServiceConfig, store: Data<RwLock<C>>) {
+    cfg.app_data(store);
+}