Fix importing webpack-merge after latest webpack release (#660)

Author: marco-c

frontend/webpack.config.js

@@ -1,4 +1,4 @@
-const merge = require('webpack-merge');
+const { merge } = require('webpack-merge');
 const common = require('./webpack.common.js');
 
 module.exports = merge(common, {

frontend/webpack.prod.js

@@ -1,4 +1,4 @@
-const merge = require('webpack-merge');
+const { merge } = require('webpack-merge');
 const common = require('./webpack.common.js');
 const OptimizeCSSAssetsPlugin = require('optimize-css-assets-webpack-plugin');
 const TerserJSPlugin = require('terser-webpack-plugin');