Pages: Improve UX when user tries to edit content on blog posts page

If you have a static front page and a separate blog posts page, it can be confusing if you change the content on the posts page. That content will never appear on the site, since the posts page content is replaced by the blog posts.

Currently, the app doesn't distinguish between a regular page and a page set as the posts page. We should clarify this in the app, either preventing the user from editing that page or clarifying what that page is for on their site. In Calypso, they block users from editing it unless it's switched back to a regular page. Screenshots of the Calypso Pages List for the page set as the posts page on a site:

See related internal report: p4a5px-2v4-p2#comment-10537

Start with the Pages List flow and the page setting that identifies a site's posts page; the issue does not name specific files or tests. Compare the existing behavior with the referenced Calypso screenshots and decide whether the page should be blocked from editing or clearly explain its purpose. Done means users no longer mistake posts-page content for visible page content.