Introduce positive messages

[Jelleas]

We have happy, sad and neutral smileys, but we can do better to motivate and support students. Even simple messages like:

Good job!
You can do it!
Well done!

We can print messages like this (or a gif in the webview) every n runs, when we find that more tests pass than before, or perhaps when progress stalls.

[cmlsharp]

We can print messages like this (or a gif in the webview) every n runs,

Ewww state (kidding).

Though we could do this without state if you relax that constraint to "every n runs in expectation" (i.e. just do it randomly with probability 1/n)