add very basic proxying

This commit adds extremely simple proxying using Rack::Proxy.  Right now
the proxy target is hard coded (and the prefix pattern to determine
proxying on), but it's a basic template.
Merge branch 'master' of https://github.com/devrieda/polly

* 'master' of https://github.com/devrieda/polly:
  add binfiles
  remove more turbolinks, setup react config