POC: SVG icons

[ppvg]

Create a proof-of-concept for using SVG icon files and mask-image, along with a script to pre-bake the icons into data-URLs.

Acceptance criteria

• (Draft) PR to the feat/manon-sass branch.
• Keep it simple.
• Don't worry about theme-specific icons (yet).
• Use Sass variables for the data URLs.
• Manon defines the .icon-*s.