Can't add a theme from private repo (because the interface changed but the instructioons didn't)

But what I think we really need is if you click the advanced button, have a thing that says “use a ssh link for private directories, OK?”

showPublicKey gets set if a ssh link is pasted in, so what might be good is if they click “advanced” and haven’t pasted a ssh link, to tell me that I are an idiot. (But in a nice way.)

https://github.com/discourse/discourse/blob/main/app/assets/javascripts/admin/addon/components/modal/install-theme.hbs#L119-L142