Registration no longer ends on an error page

Some first-time sign-ups finished on a red "400" error page, which read as "registration is broken". It was not: the account had been created, confirmed and signed in, and the app already had what it needed. The error came from the browser re-submitting the same form afterwards. That replay is now answered by sending you on to where the first submission was going, instead of an error nobody could act on.