Link previews don't honour the autoplay gifs setting

[turt2live]

similar to #6534

[lampholder]

Perhaps this one is simpler - we could just make a change in the URL preview fetching stuff to always flatten animated gifs to a single frame.

URL previews including animations seems on balance more annoying than useful

[turt2live]

I quite like the accidental feature tbh, I just happened to notice the setting isn't respected.