Redirect experimental signups to their own app on email confirmation

This PR requires [some changes in API](https://github.com/heroku/api/pull/2075) to be merged and deployed to function. Its purpose is to help us to experiment with new onboarding processes.

I've tried to be as less intrusive as possible. In order to experiment on staging instances, I've added two config settings and made them optional: nobody should need to configure them for regular Identity development, and with their default values they won't be part of the regular signup flow.

/ cc @brandur @jonmountjoy