Fix assets compression on production build

Due to a bug in uglify-js 3.4.9 compressing conditionals during asset
minization is broken, leading to #5450.

This commit disables conditionals compression until we can test it is fixed
in a future uglify-js release.

Here is a similar issue reported in upstream project:
mishoo/UglifyJS#3269

Fixes #5450

graylog2-web-interface/webpack.config.js

@@ -160,6 +160,10 @@ if (TARGET === 'build') {
           minimize: true,
           sourceMap: true,
           compress: {
+            // Conditionals compression caused issue #5450 so they should be disabled for now.
+            // Looking at uglify-js issues, it seems that the latest changes in version 3.4.9 broke conditionals
+            // compression. For example: https://github.com/mishoo/UglifyJS2/issues/3269
+            conditionals: false,
             warnings: false,
           },
           mangle: {