Bump uglifyjs-webpack-plugin from 2.0.1 to 2.1.0 in /js

Bumps [uglifyjs-webpack-plugin](https://github.com/webpack-contrib/uglifyjs-webpack-plugin) from 2.0.1 to 2.1.0.
- [Release notes](https://github.com/webpack-contrib/uglifyjs-webpack-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/uglifyjs-webpack-plugin/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/uglifyjs-webpack-plugin/compare/v2.0.1...v2.1.0)

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

js/package.json

@@ -29,7 +29,7 @@
     "css-loader": "^2.0.2",
     "karma": "^3.1.3",
     "mini-css-extract-plugin": "^0.5.0",
-    "uglifyjs-webpack-plugin": "^2.0.1",
+    "uglifyjs-webpack-plugin": "^2.1.0",
     "webpack": "^4.27.1",
     "webpack-cli": "^3.1.2",
     "webpack-merge": "^4.1.5"