REVIEW ONLY: change spinner animation to use sprites

This is for issue #3385.

I verified that this fixes problem. If we decide this animation is acceptable, then it needs a little work before merging:

1. Only small (12x12) spinner was converted. Also need to do the same for 2x spinner. Note that "2x" spinner displays as *3x* and image is *6x*, so we should optimize the image, css, and/or name.

2. My image editing foo is not so good, so the sprite sheet I made lost the transparent background (has white instead). Not sure if that's ok. Also, the transparent background *could* be part of the problem.