Raise canvas goal

[donkirkby]

In the browser version, a long code sample with a canvas goal leaves a big gap between the output canvas and the goal canvas.

• [x] Cap code sample's length at canvas height when there's a goal canvas. Enable scrolling in editor.
• [ ] Add a frame around each canvas to connect the headings with each canvas.
• [ ] Add a resize control at the bottom of the canvas frame that lets you shrink or grow all four sections.

[donkirkby]

See if Bulma tiles or boxes are useful for framing the pieces.