DEV: Remove gifsicle dependency (#10357)

Dependency on gifsicle, allow_animated_avatars and allow_animated_thumbnails
site settings were all removed. Animated GIF images are still allowed, but
the generated optimized images are no longer animated for those (which were
used for avatars and thumbnails).

The added 'animated' is populated by extracting information using FastImage.
This field was used to selectively reoptimize old animations. This process
happens in the background.

.github/workflows/ci.yml

@@ -65,7 +65,7 @@ jobs:
         if: env.BUILD_TYPE != 'LINT'
         run: |
           sudo apt-get update
-          sudo apt-get -yqq install postgresql-client libpq-dev gifsicle jpegoptim optipng jhead
+          sudo apt-get -yqq install postgresql-client libpq-dev jpegoptim optipng jhead
           wget -qO- https://raw.githubusercontent.com/discourse/discourse_docker/master/image/base/install-pngquant | sudo sh
 
       - name: Update imagemagick