Hide animated GIF stamps if "reduce motion" is on

Hide animated GIF stamps if "reduce motion" is on

Some people have a hard time dealing with motions near text they're
reading, or just don't want to be dealing with animated content
generally. The (prefers-reduced-motion: reduce) media query is active
when the reader has enabled one of the associated accessibility features
on their device. We can use that to hide the animated "stamps" GIFs
which generally have very short, fast loops.

This code currently uses ".gif" as a proxy for "is animated", which
could be improved by having some additional metadata. Additionally, it
might be nice to have alternate still versions of the gifs displayed
when the animated ones are hidden.