Extract Babel configuration to a separate package

Created by: fson

Once the monorepo configuration in #419 has landed, we should move Babel configuration to a preset package and linting rules to an ESLint config package.

This will improve the ejected experience, because we will generate less configuration and also simplify some of the underlying code, which is getting quite complex because of the differences between running inside react-scripts and in the ejected mode (as can be seen in #602).

In the long term providing smaller configuration packages makes it easier to eject without opting in to maintain all the aspects of the config. This can also make forks that reuse the config easier to maintain.