Use Atom-specific fork of React allowing selection restoration opt out

The default version of React reads the current selection when an input element is focused during reconciliation. This happens on every keystroke in the React editor, causing an extra reflow. I have a PR open at https://github.com/facebook/react/pull/1439 to add the ability to opt out of this to React-proper, but until then, we'll have to use a fork. The PR comes from my account a few weeks ago, but the fork actually lives at https://github.com/atom/react/tree/selection-restoration-opt-out. It includes changes to `package.json` so it could be published that we wouldn't want to merge into React.

I'm sure if this is the best approach structure-wise. The helper library `reactionary` depends on the requiring library having `react` available in its `node_modules`. So I published our fork of `react` as `react-atom-fork` and also published `reactionary-atom-fork` which requires `react-atom-fork` instead of `react`. Not beautiful but it works. Hopefully these fork packages are temporary.