Present a compiled tutorial

This is a bit of an omnibus PR. Many of the pieces are inter-related, so I think it makes sense to consider these all together. The major pieces are:

Improve the URL structure for project assets. The Sk.pytch environment object has a new slot which says what the project root is. All assets are loaded from under this root. In particular, the project assets like graphics and sounds come from project_root/project_assets.
The build button can now be triggered programmatically, and can optionally click the green flag also. This allows 'run the project' to build and green-flag the project.
The IDE layout is stripped down (no header bar, no menu bar) and replaced with a flex-box layout.
A tutorial can be presented in the bottom info panel.
The tutorial is loaded from one compiled with the pytch-build tool. The IDE extracts table of contents, puts in navigation links, interaction features in the patches, etc.
Minor: Bugfix for problem whereby error messages including HTML-special characters were mis-handled.

pytchlang / pytch-vm