allow developers to change manifest URLs for hosted apps and trigger re-reviews (bug 944129)

https://bugzilla.mozilla.org/show_bug.cgi?id=944129

r? @robhudson @eviljeff
please change "Please try not to change your manifest URL too frequently." to something like "If you change your manifest URL existing users won't be updated. See <a href='some-url-to-MDN'>Updating Apps</a> for more information." where some-url-to-MDN is likely to be https://developer.mozilla.org/en-US/Marketplace/Publishing/Updating_apps#Updating_hosted_apps but I'm double checking with Chris Mills.