Use first two letters of menu item as default icon
Refactor icon approach to inline SVGs instead of via CSS. This allows us to consolidate logic for both the icons and the default first two letters display, without having to do a lot of wonky CSS overrides. It also prevents the CSS from bloating each time a new icon is supported, and eliminates network requests.
Add aria-hidden to SVGs for accessibility. Hide these graphics from screen readers, as they are purely decorative.
Closes #3424744