Improve integration tests

Implement a simple web proxy in Node.js for use by Selenium-powered
browsers during integration tests. This proxy:

- Passes requests for GGA application resources through to the
  locally-running server.
- Immediately handles requests for non-essential external resources with
  an empty response
- Immediately handles requests for essential external resources with
  previously-cached responses (which are included in the
  `test/integration/fixtures/` directory)