Convert backend code to ES modules

Bundling code now with Rollup for its flat bundling and tree shaking
support. Front end code still goes through Webpack for browser support.

lib/fs.js

@@ -18,12 +18,12 @@
  */
 "use strict";
 
-const url = require("url");
-const querystring = require("querystring");
-const mongodb = require("mongodb");
-const db = require("./db");
+import url from "url";
+import querystring from "querystring";
+import { GridFSBucket } from "mongodb";
+import * as db from "./db";
 
-function listener(request, response) {
+export function listener(request, response) {
   const urlParts = url.parse(request.url, true);
   if (request.method === "GET") {
     const filename = querystring.unescape(urlParts.pathname.substring(1));
@@ -45,7 +45,7 @@ function listener(request, response) {
         "Content-Length": file.length
       });
 
-      const bucket = new mongodb.GridFSBucket(db.client.db());
+      const bucket = new GridFSBucket(db.client.db());
       const downloadStream = bucket.openDownloadStream(filename);
       downloadStream.pipe(response);
     });
@@ -54,5 +54,3 @@ function listener(request, response) {
     response.end("405 Method Not Allowed");
   }
 }
-
-exports.listener = listener;