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

package.json

@@ -47,7 +47,7 @@
     "babel-loader": "^8.2.2",
     "babel-plugin-add-module-exports": "^1.0.4",
     "babel-plugin-react-element-info": "^1",
-    "copy-dir": "^0.3.0",
+    "copy-dir": "^1.3.0",
     "css-loader": "^0.28.11",
     "enzyme": "^3.11.0",
     "enzyme-adapter-react-16": "^1.15.5",
@@ -62,8 +62,9 @@
     "jest": "^26.6.3",
     "js-sha256": "^0.9.0",
     "mini-css-extract-plugin": "^0.6.0",
-    "node-sass": "^4.14.1",
     "react-test-renderer": "^16.14.0",
+    "rimraf": "^3.0.2",
+    "sass": "^1.32.2",
     "sass-loader": "^7.3.1",
     "sortablejs": "^1.12.0",
     "style-loader": "^0.23",