Added continue_to as a kotti.register config param, and made the registr...

...ation code check for it, before preparing the confirmation email for new registrations. The confirmation email set-password link can be configured, using continue_to, to go to a URL after the new user has set their initial password. continue_to exists as a hidden property of the SetPassword schema, but it needs to be added to the appstruct before the set-password link is prepared for the confirmation email. This required overriding a lot just to accomplish this, it seemed, so having as a config parameter eases the process for this simple need.