Make homotopy.io into a progressive web app (PWA)

[calintat]

This would enable users to add homotopy.io to their desktops and use it offline.

[NickHu]

This would also allow homotopy.io to function as a phone/tablet app, for easy access on those platforms (once touchscreen support is working properly). With some more hoops, it would also allow it to be submit into the Apple App Store and Google Play Store.

[calintat]

I don't like the idea of having non-native apps on the App Store or Play Store.