Finishing registration should return to the page that made you register

When someone finishes registering with Launchpad so as to do something that requires a Launchpad account, they should be returned to the logged-in version of the page they were trying to access. (This is like bug 449, but more complex.)

For example, someone wants to report a bug in Ubuntu Firefox, and guesses that "Get Help Online..." might work. From there they click "Bugs", then "Report a Bug", and get to a login form. Not having an account, they sign up, leave the "You have been sent a registration mail" page because they use Webmail, click on the link in the message, finish registering, and end up at ... their person page. This is not where they wanted to be; they wanted to report a bug in Ubuntu Firefox.

(The other part of the original report, getting a 404 from /people/you/+filebug, is bug 4442.)