Allow each finder to have a 'signup' page

[Trello ticket](https://trello.com/c/2KUtZWCp/322-email-sign-up-page-for-a-finder-document-format-3)

Route: `/:finder_slug/signup`

Users visiting this page see copy which comes from several attributes in
the schema:

 * name: used for page title and initial summary paragraph.
 * email_signup_copy: Used for the second paragraph, free-form.
 * email_signup_url: The target for the button displayed on the page.

This allows users to sign up for alerts, using details present in the
schema for display.