Add a simple Procfile to serve static assets

[patch0]

• Sets a Procfile to use a Ruby HTTP server to serve up the app as a static site.
• Adds a heroku-postbuild script in package.json which builds both the app and the web component. This takes precedence over build in the Heroku build process. This has a special env var of PUBLIC_URL set to an empty string so the webcomponent builds properly.
• Adds a .dockerignore to make the docker build context smaller for faster local container rebuilds
• Adds an app.json just to make the Heroku deployment a bit more in-keeping with other apps.

So this means that the review app is now served from https://editor-ui-pi-use-webric-wzk77h.herokuapp.com/ with a basic Ruby webserver, and then the webcomponent is at https://editor-ui-pi-use-webric-wzk77h.herokuapp.com/web-component.html 🎉

The main editor showing the sense hat working:

And the web-component:

Not sure this is great for production use, but it'll do.

[raspberrypigithubbot]

• CircleCI build #655
• Test coverage: 62%

[raspberrypigithubbot]

• CircleCI build #656
• Test coverage: 62%

[raspberrypigithubbot]

• CircleCI build #658
• Test coverage: 62%

[raspberrypigithubbot]

• CircleCI build #659
• Test coverage: 62%

[raspberrypigithubbot]

• CircleCI build #660
• Test coverage: 62%

[loiswells97]

Looks okay I think 👍