Add -layers 'optimize' flag to animated gif convert cmds

### WHAT

When converting animated gifs, pass the `-layers 'optimize'` flag. 

### WHY 

This optimizes the frames of the animated gif, making the file size smaller.

Currently -- without this change -- an optimized animated gif will be saved with each frame at the full size of the first frame. We noticed this happening when we found a 200kb animated gif (pre-optimized) with 40 frames that we'd saved with Paperclip. Saving it in its original size, the converted file ended up being 5mb because it has upsized all of the frames.