Switch to using SVG sprites for icons, instead of using a combination of icon fonts and inline SVG elements.
This means there's a single icons.svg file containing all icons, and icons are referenced like
`<svg><use href="/icons.svg#arrow-alt-up"/></svg>`.
This should make icons more efficient since a) it replaces an 80kb icon font file with a 8kb SVG
file and b) it replaces repeated inline copies of the same icon with references to the sprite sheet.
This most notably affects the upvote and downvote buttons.
This also updates icons from Font Awesome 5 to Font Awesome 6, which fixes a few minor icon
alignment issues in some places.
ref: https://fontawesome.com/docs/web/add-icons/svg-sprites
