Make PUBLIC_URL environment variable available at build time.

Created by: t-my

Previously PUBLIC_URL was read from package.json homepage entry. Webpack DefinePlugin then processed the PUBLIC_URL to be available at runtime. However setting the environment variable should have precedence over configuration. Currently setting the PUBLIC_URL has no effect. This can be very confusing for a developer.

Fixed by looking into process.env.PUBLIC_URL before assigning a value from package.json.

Test plan:

$ export PUBLIC_URL=testing && npm run build

You should immediately see from the output if create-react-app picked it up.