Add remaining meta data to case study format

This adds the last two bits of meta data (related policies and worldwide priorities) to the case study schema.

**Note that this includes commits in github.com/alphagov/govuk-content-schemas/pull/1. Assuming that is merged first, this pull request will consist of the final commit only**.

Story: https://trello.com/c/16aW6TD4/73-add-remaining-meta-data-to-case-study-format