Update icons to use latest set, remove icons that are no longer used

**Before:** PNG sprite size: 21,192kb, SVG images in /images/global: 120
**After:** PNG sprite size: 17,697kb, SVG images in /images/global: 78

This PR touches a few bits of the commercial components (search input, CTAs and buttons where I have consolidated various icons into one) and discussion (when I’ve updated the reply icon, deleted some redundant SCSS… although I noticed quite a lot more!)

There are still quite a few more images that can be removed/consolidated, but this should be a good start.