Make multi-user and app sharing work for families

[DasSchweinhund]

It's my understanding that the current, experimental implementation of these features will require each account to have a Facebook account. If you're, say, a parent, trying to organize time on a single Quest, you'll need accounts set up for children, often younger than 13, which makes the current implementation useless unless we're looking to violate Facebook's user agreement. I'd suggest giving users a way to set up designated kid's accounts, maybe similarly to how kids' messenger accounts can be set up.