Added TypeScript for frontend

Added type definitions for components, functions, data
Added guards for network responses
fixes #8

front/src/pages/UserPage.tsx

@@ -0,0 +1,9 @@
+import { Link } from "react-router-dom"
+
+function UserPage() {
+    /* TODO */
+
+    return <h1>For Yet Go <Link to="/">Home</Link></h1>
+}
+
+export default UserPage