Optimise SVG icons

* Removed Sketch specific attributes from expand icons
* Simplified a few icons with `<defs>` used only once inside SVG document
* Consistent attribute order, i.e. `<svg xmlns="http://www.w3.org/2000/svg" width="X" height="X"><path fill="#XXX"…` I might be overly hopeful in thinking this will reduce CSS file sizes — how similar are strings once the SVG syntax has been converted to base64?

Also:
* Updated clock hands icon to use new icon style
* Updated show-all-comments to new icon style

…and no deletions :-)