randomize sprite positions when added from image file or from library (through 'choose from library' and 'surprise')

Toward LLK/scratch-www#2296

This allows us to take you to any destination when clicking the logo, or to do something else, or nothing at all.

If no handler is provided, the logo will not be clickable.

Co-Authored-By: towerofnix <9948030+towerofnix@users.noreply.github.com>

Resolves #3950.

Unfortunately the design of scratch-storage doesn't make having multiple distinct POST urls easy at the moment. So for the time being, just make this request ourselves.

We are emitting targets update because the event is disabled in player mode, but gets re-enabled and re-emitted when switching back. The API on that function was changed to allow events to be marked "not project changes"

That is, as the default value for the input, not the placeholder.

Adding relevant EV3/WeDo2 tutorial vid links, fixing up some json, removing some prop validation, etc.