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. 
Yeah, this is how the mockups were, and the tutorial **does** go away as you start to click and do stuff in appmaker. I agree that it feels weird though. Maybe a "done" button would help?