Intro tutorial

Adding `tutorial` as a query parameter to the designer url will launch a brief walkthrough. The walkthrough HTML content is defined in `public/tutorial/intro.html` and the walkthrough itself is defined in `public/tutorial/intro.js`. 

The content is not yet complete or accurate, but the basic mechanism is ready for a first review. 
I just copy/pasted from `changeEditableTab` in js/modes.js, which I'd prefer to expose somehow and reuse so that if the "show a new tab" logic changes, the tutorial doesn't have to care. I should have thrown in a FIXME or something.