Contribute Operational Transformation to Scratch Editor, and make collaborative Scratch project per world instead of user

[vorburger]

https://en.wikipedia.org/wiki/Operational_transformation

https://youtu.be/qz2gDUJklWk

@edewit Aerogear?

[vorburger]

@edewit and I spoke about this again today, and we now see this as a natural next step extension of #50.

Basically what this would need is "just" to constantly & frequently (<1s)...

1. autosave
2. server-side "magic Scratch project JSON merge"
3. "push back from server to client"

Let's do this AFTER we have fully finished #50 and other BYCTWD milestone issues!