Implement the reviewed UI icon system and durable review workflow (#853)

Thanks @Nathan-Dane.

.github/workflows/ci.yml

@@ -119,6 +119,8 @@ jobs:
         run: cargo fmt --all -- --check
       - name: Asset attribution
         run: cargo xtask assets
+      - name: UI icon standards and review tool
+        run: cargo xtask icons
       - name: Licences
         run: cargo xtask licences --check
       - name: Dependency layers