allow homepage to provide a full URL in production

Created by: redbmk

Fixes #8813

To test this I first did went into packages/react-dev-utils in this branch and ran yarn link. Then in another folder I ran:

npx create-react-app testing
cd testing

Next I added "homepage": "https://example.com" to package.json and ran yarn build.

The output contained this:

The project was built assuming it is hosted at /.
You can control this with the homepage field in your package.json.

I then ran:

yarn link react-dev-utils
yarn build

which contained this output:

The project was built assuming it is hosted at https://example.com/.
You can control this with the homepage field in your package.json.