Redirect to login, if confirmation token was not found

Fixes #647

Currently, clicking the link in the registration confirmation email more than once, leads to a NotFoundHttpException. For a better user experience, user will now be redirected to the login route instead.

Alternatively to this PR, one could think of adding a parameter, which - if set - describes the route to be redirected to. If not set, the old behaviour with the NotFoundHttpException remains.