Add MakeCode watermark in workspace for embedded tutorials

[kiki-lee]

Now that others will be hosting tutorials in their web pages (with no top nav bar) we need a way to showcase that people are using MakeCode Arcade without having it look like we wrote the tutorial. The top suggestion is to add a Microsoft MakeCode Arcade watermark into the workspace.

Other options could still be considered.

[kiki-lee]

Another reason we should have the Microsoft MakeCode logo in more places. It's too easy to hide the top bar!

[kiki-lee]

Still a good idea