use webpack middleware to avoid stale bundles

[collin]

Replaces the multi-process bundle approach with webpack-dev-middleware

This is a common annoyance for students and shows up as this sequence while debugging:

• 1 Student makes change to front-end code and save file.
• 2 webpack starts bundling
• 3 Student reloads web browser, loading stale bundle.js. Student debugs against old code, possibly drawing incorrect conclusions about whether their changes were correct.
• 4 webpack finishes bundling
• 5 Student goes back to editor and makes more changes.
• 6 Cycle repeats.

I've regularly stepped into Sr Phase help tickets where students were in this cycle and did not realize it, having wasted quite a bit of time and having lost a lot of confidence in their sanity.

Using webpack-dev-middleware would increase a delay and the cycle would go like this:

• 1 Student makes change to front-end code and save file.
• 2 webpack starts bundling
• 3 Student reloads web browser, server blocks until bundle is complete, fresh bundle is served.

This PR includes an explanatory comment for students.

  //   This middleware will match requests to GET /bundle.js
  //   An advantage to using this middleware is if webpack is
  //   in the middle of a compilation the request will not
  //   return content until the fresh bundle is availble.
  //
  //   In production, the bundle will be generated and stored in the
  //   public/ directory.