app-start: on select Demo Game present it

[rudifa]

app-start: on select Demo Game present it
index.html: remove href="game.html"
rename static/game.html -> static/game-UNUSED.html
rename components/app-game.js -> components/demo-game.js

This PR demonstrates how to instantiate a game component (<demo-game>) in the <app-start> component.

Related issues to discuss:

• I would like to port the code from branch add-hexagon-groups onto the main branch and add it as another game, say <hexa-game>
• I think that the ported code should go into per-app directory, something like this

components/
    app-start.js
    games/
        demo-game/
            demo-game.js
            other.js
        hexa-game/
            hexa-game.js
            more.js
        yetanother-game/
            yetanother-game.js
    utilities/

• Next, we could structure each game directory as a local component