turn the sandbox demo into a react project

[benma]

You can run the sandbox demo using:

make builddemo
make servedemo

The demo code lives in sandbox/.

This subproject is messy:

• it does manual DOM manipulation
• make builddemo takes minutes because it webpacks the big bitbox02-api-js
• there is no hot reloading
• hard to maintain, hard to read

It would be great to turn it into a react project with nice components. Webpack should also be configured to do code splitting, so the big bitbox02-api-js is not bundled in the main bundle but copied to a separate one. Here is a commit in adalite that does the same: vacuumlabs/adalite@aa2a8f9