[low priority] Add collaboration client to multiplayer, hidden behind `collab=1` flag

[eanders-ms]

This is the real-time collaboration (multiplayer paint) experiment I added to multiplayer. It's hidden behind the collab=1 url param. Multiplayer works the same as before when this flag is not present.

Having an "always available" collaboration system at the ready will enable experimentation in ways not possible before, so I'd like to have this code merged to master to serve as a reference.

A lot of the changes here are from prettier. I extended the max line width for readability.

Try it: https://arcade.staging.pxt.io/app/b6dacd14ffe7e83a637d86a596f8821d69e9988a-6194d61c1c--multiplayer?collab=1#