move proxy config to YAML file

This commit changes the proxy's target and prefix from being hard coded
to being read from a YAML file.
Merge branch 'master' of https://github.com/devrieda/polly

* 'master' of https://github.com/devrieda/polly:
  separate client/server apps into their own directories
  bootstrap in basic hello world in react
  disable cors before filters
  fix boiler plate refs to polly
  add polly lti engine