fix: www/build_common/package.json & www/build_common/yarn.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-POSTCSS-5926692

Author: snyk-bot

www/build_common/package.json

@@ -8,9 +8,9 @@
         "@babel/plugin-transform-runtime": "^7.4.3",
         "@babel/preset-env": "^7.4.3",
         "@babel/runtime": "^7.4.3",
-        "autoprefixer": "^9.5.1",
+        "autoprefixer": "^10.0.0",
         "babel-loader": "^8.0.5",
-        "css-loader": "^2.1.1",
+        "css-loader": "^5.0.0",
         "file-loader": "^3.0.1",
         "html-webpack-plugin": "^3.2.0",
         "import-glob-loader": "^1.1.0",
@@ -28,7 +28,7 @@
         "mini-css-extract-plugin": "^0.6.0",
         "node-libs-browser": "^2.2.0",
         "null-loader": "^1.0.0",
-        "postcss-loader": "^3.0.0",
+        "postcss-loader": "^4.0.0",
         "pug": "^3.0.1",
         "raw-loader": "^2.0.0",
         "style-loader": "^0.23.1",