Updated for 0.7.1

[delapuente]

The main goal of this PR is to gather feedback. During the update, I've found the following issues:

1. The outline component does not work with A-Frame 0.7.1
2. The virtual city glTF 2.0 model is partially broken (some ships are displayed as cubes)
3. Add tracked controls Glitch breaks Nightly, Quantum and Chrome (it stops loading). I've removed it from this version but my intention is to find the problem and restore it.

I've added two new sections:

1. "Cast shadows" extends the lesson "Compose with Entity-Component"
2. Augmented Reality with A-Frame and AR.js

[delapuente]

@ngokevin would you mind to update glitches to use A-Frame 0.7.1, please? I think all of them work except the one with the outline component.

[Utopiah]

@delapuente just tried https://github.com/takahirox/aframe-outline own example and worked with 0.7.1