[webpack-config] Implement public path

[EvanBacon]

Why

To make this work -> https://docs.expo.io/versions/latest/distribution/publishing-websites/#github-pages

How

Added the ability to publish to a publicPath based on the homepage field of the package.json.

Test Plan

1. expo init
2. Add URL

// package.json
+   "homepage": "http://evanbacon.github.io/gh-pages-expo-web-deployment-test",

3. expo build:web && yarn gh-pages -d web-build

[cruzach]

Deploying to Github pages worked, but if you have homepage set in package.json there are issues with the other deployment methods (Now & Netlify)

Is it possible to still have the public path url work for all the methods, even if homepage is set for gh pages? Or should we just make it clear to not have that set if you're building for not gh pages

[EvanBacon]

@cruzach setting process.env.WEB_PUBLIC_PATH="gh-pages-expo-web-deployment-test" should work for that. Could we work this into the docs somehow? Also we need to update the docs to reflect the correct build path:

- build
+ web-build

[cruzach]

Yeah that worked, I'll update the docs