Improve development setup for embed

[supermacro]

The embed currently requires changing a hard-coded embedAppOrigin variable in the demo/index.html page based on whether we are running locally or deploying to production.

Additionally, the developer needs to manually cd into the /demo directory and run a web server to serve this demo host index html page and subsequently run another process to serve up the elm embed application on a separate port.

Additionally, the developer needs to have elm already installed.

Lastly, the node version is not pinned down.

This all seems a little annoying / cumbersome.

Desired Outcome:

• There should be just one command to do all this work for a developer, namely npm install
• There should be environment variables set in demo/index.html so that we don't have to hardcode values and not have an error prone deployment process