Examine and improve cache clearing for CPTs

For example, the case study listing page should have its cache cleared when a new case study is published.

I assume that the blog post listing and homepage have their cache cleared when a new blog post is published but we should double-check.

Pantheon has a way of selectively clearing certain pages when a content type is updated. Read about "surrogate keys" in [the Advanced Page Cache plugin](https://wordpress.org/plugins/pantheon-advanced-page-cache/).

Start by reviewing the content-publishing and cache-clearing paths for case studies, blog posts, the homepage, and listing pages. Read the Advanced Page Cache plugin documentation on surrogate keys, then verify that publishing each content type selectively clears the pages that depend on it; done means the relevant cached pages refresh without unnecessarily clearing unrelated pages.

Written by the indexing model from the issue text.