Commits building up to the refactored UI

This pull request consists of a series of commits that build up to the fully refactored version of AppRTC as at [apprtc-m.appspot.com](https://apprtc-m.appspot.com).

The first commit is a minimal version of the new UI without any of the following: 
· analytics
· header/icons
· camera change
· sharing (except link)

Each subsequent commit will add these features.

I see what you mean.

I've refactored a bit.

FWIW might also be worth refactoring out `hideDiv()` and `showDiv()` functions at some stage, to avoid the repetition of stuff like `fooDiv.classList.add('active')`.