Enable Webpack tree shaking

[willopez]

Resolves #351
Impact: minor
Type: feature

Issue

Wepack tree shacking was not enabled due to the following setting being in our .babelrc,
modules: commonjs

This PR removes that setting to effectively enable tree shaking(dead code elimination)

Solution

Follow NextJS' example when using a custom web server: https://github.com/zeit/next.js/blob/canary/examples/custom-server-express/server.js

Testing

1. Rebuild app
2. Complete the checkout flow, both login and as an anonymous user.
3. Verify there are no errors

[rosshadden]

Okay this makes sense. So there are essentially two runtime contexts. One is the startup code (the server) and the other is the Next.js routes and associated code. You had to change everything in the server context, everything they import, and everything they export to use commonjs imports and exports, and then the Next.js context can be ES modules because Next imports them itself through webpack.

Nice work.

[rosshadden]

Looks good to me. I tested it as well, and had no issues. Also fixed merge conflicts that just appeared a few minutes ago.

[spencern]

@willopez @rosshadden it looks like there are two lint errors that are being flagged in CI.

[rosshadden]

They might have arisen from the merge conflicts. I'll take a look.

[rosshadden]

Yeah my fault. I saw session in both sides of the diff and didn't notice express changed to cookie upstream. Fixed.