Move Prismic Linting over to Github actions, occur once a day only

[rcantin-w]

Prismic linting is a Buildkite task that runs on every branch's every commit that is pushed to Github. It's also triggered every 20 minutes of office hours. I believe that it is not necessary. The linting report get consulted at most once a day, so putting this on a once a day schedule instead would be a better use of resources.

The script currently scours the Prismic data for these checks;

• Look for eur01 safelinks. These occur when somebody has copied a URL directly from Outlook and isn't using the original URL.
• Look for preview.wellcomecollection.org links.
• Look for broken links to interpretation types on events.
• Contributor links that don't begin with http or https will be treated as relative URLs on the front-end, e.g. www.example.com becomes wc.org/articles/www.example.com.
• Stories without a Promo image should be rare as they are considered required now, but older articles won't necessarily have them.
• Contributors have a sameAs field which is used to generate links to their pages elsewhere, e.g. social media or organisation websites.

I don't think any of this requires the script to be run that often.

Had a chat with @kenoir, he's expressed interest in taking it away from Buildkite into a Github action. This would also mean having to set up appropriate AWS access for Github actions in the wc.org repo, which would probably be useful in the future anyway. That has been done before in other repos so there is a reference available.