Initial Mebbling hub implementation

lib/http.ts

@@ -0,0 +1,6 @@
+export function externalUrl(request: Request, pathname: string) {
+  const current = new URL(request.url);
+  const host = request.headers.get("x-forwarded-host") || request.headers.get("host") || current.host;
+  const protocol = request.headers.get("x-forwarded-proto") || current.protocol.replace(":", "");
+  return new URL(pathname, `${protocol}://${host}`);
+}