feat: Purge Cloudflare cache

[mgax]

Fixes https://github.com/ietf-tools/wagtail_website/issues/361.

Additional setup

• Generate a Cloudflare API token (not API key) with permissions to purge cache: https://docs.wagtail.org/en/stable/reference/contrib/frontendcache.html#cloudflare. Set it as the CLOUDFLARE_CACHE_PURGE_BEARER_TOKEN environment variable.
• Retrieve the Zone ID of the Cloudflare zone for www.ietf.org. Set it as the CLOUDFLARE_CACHE_PURGE_ZONE_ID environment variable.
• Change the Wagtail site (in the admin, Settings -> Sites) so that its port is 443. This tells Wagtail to generate https:// URLs. Otherwise, it will try to purge http:// URLs, which won't work.
• To test that cache purge works, run curl -s https://www.ietf.org -I | grep cache. It should print cf-cache-status: HIT for a cached page and cf-cache-status: MISS after it's purged.

Cache purge rules

• When a page is published or unpublished:
  • Purge this page.
  • Purge this page's parent page.
  • Purge any pages which reference this page (e.g. with a link in a rich text block, or the "Promoted Events" list on the Meetings page).
  • If a MainMenuItem references this page, purge the homepage. (Technically we should purge all pages, but that's potentially expensive, as they have to be enumerated, and it seems overkill.)
• When a MainMenuItem is saved or deleted:
  • Purge the homepage.