DEV: use native lazy load on avatars in topic lists and posts (PR #14804)

This PR adds the loading="lazy" attribute to avatars in topic lists and in posts (poster avatar).

Avatars in the topic list are small, so bandwidth is not really the concern. It’s more about prioritizing the requests for the ones that are already in the viewport.

This should be handled by the browser natively. Browser support is currently at 75%.

Safari has this behind a flag. If disabled, it will simply ignore the attribute.


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