Fork should fork

[Rich-Harris]

At the moment, 'fork' actually creates an entirely new gist. Might be better if it actually preserved the fork information, especially if we start using the REPL as the foundation of a component marketplace

[Conduitry]

I think there should still be a separate 'save' option. The way the REPL's set up now you always have some gist loaded. Not sure what the best UI for this is.