React app rewire ts jest

[christensena]

NOTE: Merge #7 first

This adds ts-jest rewire. It is an independent package as the new syntax for rewiring jest config is separate.

From the README:

react-app-rewire-ts-jest

Add Typescript testing with jest to a react-app-rewired config.

/* config-overrides.js */

const rewireTsJest = require('react-app-rewire-ts-jest');

module.exports = {
  jest: function override(config) {
    // ...
    config = rewireTsJest(config);
    // ...
    return config;
  }
}

[icopp]

See the comment on the other PR. I'll see about extracting this into its own repo when we next do our internal infrastructure update stuff.