Don't attempt to load previews for gifs hosted by imgur. ...

imgur's gif previews are not animated. Without this change, it is
difficult to enjoy gif-heavy subreddits because anything hosted on imgur
fails to animate.
Improved handling of edge cases for loading GIFs instead of there static preview image.
Added compiled JS file based on JS code from commit 4ffc401e81720d815330f025bc5716b03bdb1d52.