Kiosk - when game is compiling/loading we should show a loading spinner

[riknoll]

It's all in the title!

[eanders-ms]

We do show the simulator's "loading...", but it's black on very dark gray 😕 . Will see if we can fix it in css.

[abchatra]

Dupe of https://github.com/microsoft/pxt-arcade/issues/5407