publishing now has a persistent URL

Note that this was not tested against remixing with a different persona account. It works quite well (publish an app, menu has App URL, switch to not-published app, App URL does not show in the menu. Switch back, it shows up again). This may still need testing against user A publishing an app X, and user B clicking "remix" for that app X. Pretty sure it won't show the original app's URL now, since that user doesn't have the same mongoose entry for {username, appname}.