Lazy loading of attachment thumbnails

[DangerOnTheRanger]

Now that loading="lazy" is supported among a decent chunk of web browsers, I think it would be good for Maniwani to use this attribute in areas where attachment thumbnails are used - the catalog, firehose, threads, and the board index.

Note that among browsers, at least Firefox disables lazy loading if JS is not enabled. I consider eager loading in that case to be a graceful enough degrade, so it should be okay. A JS fallback for Safari would be nice to have, but not necessary.