fix(ui): change biuld optimization for parralel option

dropping it to 2 for the time being. seeing a lot of failures in CI when
parallel is set to true.

issue aligns with webpack issue: https://github.com/webpack-contrib/webpack-bundle-analyzer/issues/214#issuecomment-458227171

ui/webpack.prod.ts

@@ -35,7 +35,7 @@ module.exports = merge(common, {
     minimizer: [
       new TerserJSPlugin({
         cache: true,
-        parallel: true,
+        parallel: 2,
         sourceMap: true,
       }),
       new OptimizeCSSAssetsPlugin({}),