toggle rotation on click

[stevegt]

The important bits that address #76 are in App.js. Everything else here and in #78 shows what I needed to do to get the build working.

• closes #76
• adds package.lock for versioning of npm modules, yarn version in package.json
• generates more complex (chunked js) structure for assets
  • assumes build is via make build-with-assets in master branch, or equivalent
  • Makefile in stevegt's fork at commit 3a47b3dd293f98dd6471996d5d41dda7c6f984a8
  • also see #77 and #78