When Jetpack's image CDN is active, core calls to retrieve images return an image served by the CDN.
Since Fediverse instances usually fetch and cache the data themselves, we do not need to use the CDN for those images when returned by the ActivityPub plugin. In fact, we really do not want that to happen, as Fediverse instances may get errors when trying to fetch images from the CDN (they may get blocked / rate-limited / ...).
Let's hook into Jetpack's CDN to avoid that.
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