Add signup route copy and url

This registers a fixed route for each finder with panopticon for `/:finder_slug/signup` when the `rake panopticon:register` task is run.

In addition, it adds two extra attributes to the schema -- `email_signup_copy` and `email_signup_url` explained more in the [finder-frontend PR](https://github.com/alphagov/finder-frontend/pull/74)

TODO:

- [ ] Add copy for all other schema types, not just CMA cases.