Move more things to react-dev-utils

Created by: ForbesLindesay

I want to make react-scripts easier to fork/extend. I think the way to do that is to move as much as possible into react-dev-utils so that react-scripts can be a smaller package that changes less frequently. Then you can always just update the react-dev-utils to get most new features in react-scripts.

This should also make ejecting much less painful, as ejected projects still depend on react-dev-utils

My proposal is to move:

1. config/webpackDevServer.config.js - This can just take publicPath and appPublic as additional options
2. config/polyfills.js - this can just be moved as is - do we want to include it in what gets ejected still, or just reference it from react-dev-utils? I feel like just referencing it probably makes the most sense.
3. config/env.js - This will need to take dotenvPath as an argument, and delete require.cache[require.resolve('./paths')]; will have to be done elsewhere.
4. each of the three jest transforms - these can just be moved as is, and then they won't need to be ejected either.
5. scripts/utils/createJestConfig.js - this will just need to take paths.testsSetup and paths.appPackageJson as arguments.

I then think we should add a pathUtils module in react-dev-utils, containing resolveApp, getPublicUrl, getServedPath and resolveOwn. This should simplify config/paths.js quite a bit.

At this point, I think we should re-evaluate what else needs moving. If nobody objects, I'll start submitting pull requests for these changes.