Snowpack instead of Rollup & OpenWC

[rudifa]

Replaces Rollup with Snowpack for building the app, as well as the Open-WC es-dev-server and app template with Snowpacks own dev server & app template.

[rudifa]

@jdhr @olange Please test and review this commit and merge into the master if OK

• About Snowpack.

• I added the apollo client arcade/app-snowpack along with the start:app-snowpack command in arcade/package.json so that now npm start starts the engine and both clients, which run on localhost:8000 and localhost:8080 respectively.

~/GitHub/js-gong-fu/arcade(rf-add-app-snowpack|✔) % tree -L 2 -I node_modules
.
├── LICENSE
├── README.md
├── app
│   ├── README.md
│   ├── custom-elements.json
│   ├── dist
│   ├── index.html
│   ├── package-lock.json
│   ├── package.json
│   ├── rollup.config.js
│   └── src
├── app-snowpack
│   ├── README.md
│   ├── babel.config.json
│   ├── package-lock.json
│   ├── package.json
│   ├── public
│   ├── snowpack.config.js
│   └── src
├── engine
│   ├── Dockerfile
│   ├── index.js
│   ├── package-lock.json
│   └── package.json
├── package-lock.json
└── package.json

• I ported the client code from arcade/app, simplified the file structure and used decorators

  @customElement('arcade-app')
  export class ArcadeApp extends LitElement {...}

[rudifa]

@jdhr @olange Please test and consider for merging into the master

In the latest 3 commits (be0f3a7 936f919 769f6de) I added the first mutation on ArcadeBook which lets the user add 'like' (as many times as she wants).