Use a more sophisticated template for end-to-end testing.

Created by: wdhorton

This addresses #706 (closed). The features that I've included in the kitchensink template so far are:

• JSX (technically the normal template was already using it for divs and stuff, but this tests it for custom components, and passing props)
• Flow
• NODE_PATH
• generators
• async/await
• class properties
• custom env variables via shell and .env file (though with the current testing setup it only tests that these don't make the app crash on build and start, rather than whether they actually get through)

Test Plan: ran e2e.sh, confirmed it used the kitchensink template, and it ran to the end with no errors.

Let me know if there are any other features I should add in, or if I should show more advanced usage of the ones listed above.