Site names now use hyphens - and old underscore links redirect

Site subdomain names now use hyphens (my-app.playcode.site) instead of underscores. Underscores are not valid in web hostnames, so my_app.playcode.site could never load in a browser - the editor just didn't tell you. Every published site with an underscore name has been renamed to its hyphen twin automatically, old underscore links now redirect permanently to the new address, and your numeric address keeps working as before.