Replace GraphicsMagick with Sharp

[claviska]

This is a breaking change and removes some features of DynamicImages (mostly filters), but it's necessary for performance reasons. GraphicsMagick isn't nearly as performant as Sharp, which shows when an image-intensive page loads without cache.

Sharp is built for speed and also supports WEBP and SVG. It doesn't have certain features such as flip, blur, and various filters, but I don't see those being used that heavily in themes anyway. The performance boost is well worth the tradeoff.

• Remove gm and add sharp
• Update dynamic images module
• Update docs