Register email signup artefact for each finder with Panopticon

For each finder, we want to register an additional artefact of kind `finder_email_signup` so that users can sign up for email alerts in finder-frontend for each finder. This kind was added in https://github.com/alphagov/govuk_content_models/pull/226

[Trello ticket](https://trello.com/c/2KUtZWCp/322-email-alerts-sign-up-page-for-a-finder-document-format-3)

- [ ] Clear copy in metadata files with PO
- [ ] Merge https://github.com/alphagov/panopticon/pull/212
- [ ] Deploy panopticon
This should match the prototype:

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