Bring up-to-date with whoops/reagent

I'm doing this as a pull request for the sake of discussion.

This brings us up to date with https://github.com/Whoops/reagent 0.4.3. In other words this gives us React 11.1 and fixes some warnings but does not include cursors, since I think the decision was to implement them as a separate project under the org. I think this is probably our best starting point.

This also namespaces it as reagent-project/reagent.core (though if we can get access to reagent on clojars from @holmsand we'll probably want to change it back to reagent/*) and updates the URL to point to the org project page.

Does that sound good @seancorfield  @yogthos?