Playground block: Add ability to maximize code sample

[brandonpayton]

Requested here by @jasmussen as "Button to maximize the code sample."

Alternately, instead of maximizing the code sample, maybe we could make it possible to collapse both the Preview pane and the Code sample pane and toggle between the two, at least in side-by-side mode. When the Preview pane is collapsed, the code editor would take the remaining space, and vice versa.