Create basic Contact Content Block

[Harriethw]

https://trello.com/c/Fa5E0qo6/1031-tech-task-create-basic-contact-content-block-schema-in-publishing-api

• add initial schema for a Content Block Contact
• add content_block_contact to allowed types
• add content_block_contact to expansion rules

-- ⚠️ This repo is Continuously Deployed: make sure you follow the guidance ⚠️

This application is owned by the publishing platform team. Please let us know in #govuk-publishing-platform when you raise any PRs.

Follow these steps if you are doing a Rails upgrade.

[Harriethw]

Blocked by changes to the content data API here https://github.com/alphagov/content-data-api/pull/2298