Tilt is broken on iOS13

[schanzer]

See https://dev.to/li/how-to-requestpermission-for-devicemotion-and-deviceorientation-events-in-ios-13-46g2

If we want this data, we need to ask for permission in response to a user event. In WeScheme, I've implemented this check when the user clicks the run button for the first time. That means an unnecessary dialog when the user tries to run a program that doesn't need orientation events, but I think it's worthwhile.

[schanzer]

Arg, I should have brought this up on the call with Ben and Joe today. This is actually going to be increasingly urgent as more teachers use CPO for Algebra. It's also a relatively small fix.

@jpolitz or @blerner , would either of you be free for another peer-coding session to get this added to production? Or is it easier if I point you to the fix on WeScheme?

Basically, we just need to add an iOS-specific check to the Run button.