Rework frontend cache purging request model and signals

[chigby]

Follow up from #1773 (comment), previously discussed in #1155

Our existing frontend cache purging code connects page events (such as publishing and deleting) to requests to Cloudflare to purge the cache for related pages. This code could be made more efficient by using purge batches: Wagtail's way of purging multiple pages in a single request (we are currently sending one request for each page we want purged).

We could also be using the page_unpublished signal instead of the post_delete signal. When a page is unpublished, it disappears from the public site (so related pages should have their caches purged), but it does not send a delete signal. On the other hand, when an editor selects "delete" from the admin UI, it will send an unpublish signal prior to being deleted. So using this signal as a way of knowing when to clear the cache is more correct based on how wagtail expects to be used.