Bump webpack from 4.17.1 to 4.20.2 in /js

Bumps [webpack](https://github.com/webpack/webpack) from 4.17.1 to 4.20.2.
- [Release notes](https://github.com/webpack/webpack/releases)
- [Commits](https://github.com/webpack/webpack/compare/v4.17.1...v4.20.2)

Signed-off-by: dependabot[bot] <support@dependabot.com>

js/package.json

@@ -29,7 +29,7 @@
     "karma": "^3.0.0",
     "mini-css-extract-plugin": "^0.4.2",
     "node-sass": "^4.9.3",
-    "webpack": "^4.17.1",
+    "webpack": "^4.20.2",
     "webpack-cli": "^3.1.0",
     "webpack-merge": "^4.1.4"
   },