When in the timeline an image bubble with animated=false was displayed and
you tapped on an image, it would attempt to show the thumbnail with animated=true.
This thumbnail, of course, was not cached, thus not showing the cached image, and
actually downloading the thumbnail multiple times. This PR fixes that by
first checking if the animated=false thumbnail is cached, and if so, display
that while the final image is loading.
e7b4c6f848