step mode for level builder, plus example levels

This PR does two things
(1) Add a new MazeStep script that has some example step levels.
(2) Allow curriculum writers to specific whether levels have a step button, run button, or both.

The second of these may make the first unnecessary, and it might be silly to check this new script in when we don't really intend to ship it. However, my thinking was it might be nice to have a way for people to play around with step functionality and provide feedback immediately. Thoughts?

Also, look closely at the level builder stuff as I haven't written a lot of dashboard code.