Update to react-router-4

[tofumatt]

This will be a fair bit of work but filing here as an issue on our radar, so we can close the Greenkeeper PRs and track it here.

[kumar303]

It took me a while to wrap my head around this one but now I've hit a road block. redux-async-connect relies on the old router API and since it declares react-router as a peer dep, it doesn't have its own isolated copy.

We could fork it and hack package.json to declare it as a dep but eyeballing what it does with ReactRouter tells me that it still may not work. It looks like we need to replace redux-async-connect with redux-saga so let's wait on the router upgrade until then.

[kumar303]

Here's my patch that was very broken. I doubt we'll need to reference it but maybe.
react-router-4-upgrade.patch.txt

[kumar303]

Here is the migration guide: https://github.com/ReactTraining/react-router/blob/master/packages/react-router/docs/guides/migrating.md