Photon: Limit to originating domain

For popular posts, scrapers will often pick up the entire post, including referencing the original `img` tags. For Photon-enabled sites, this means the scrapers are effectively hotlinking from Photon.

Suggested: Disallow requests from non-originating domains or make an option to do so.

Originally requested in WP Core Slack via private message.

Start by locating the Photon request-handling entry point and trace how requests from domains other than the originating site are identified. Reproduce the scraper hotlinking scenario and define whether completion means blocking those requests or providing a configuration option; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.