Making no fork repls official

cont. discussion from

Describe your feature request
Allow an official setting to prevent others from forking the repl
What problem(s) would this feature solve?
It would prevent others from stealing your code
(And it would be easier to do since it is official)
Explain what you were trying to do when you came across the problem leading to this feature request
I saw a forum post about no fork repls and saw it seemed like it took some code to do. Not everyone knows about that thought so adding a button would make things easier