Build process improvements

- Ensure static directory is always fully regenerated, to ensure
  reproducibility
- Exclude Photoshop and Illustrator assets from the static directory
- Switch from node-sass to sass, because not everyone has a C compiler
  available or can easily work with node-gyp
- Fix many sass compilation errors and warnings

src/scss/components/_debug.scss

@@ -8,7 +8,7 @@
 	z-index: 96;
 	padding: 10px;
 	background: rgba(100,100,100, 0.8);
-	color: colour(white);
+	color: colour('white');
 	pointer-events: none;
 	width: 220px;
 	max-width: 98vw;
@@ -21,4 +21,4 @@
 			padding-top: 0;
 		}
 	}
-}
+}