Build translations with webpack

Convert the `po2json` script into a webpack plugin, and integrate the
(now trivial) po.empty.js template.

This is the last step for building the entire dist/ directory with
`npm run build` (i.e. a tool that web developers are familiar with),
and not having a split webpack+make toolchain any more.

webpack.config.js

@@ -1,11 +1,14 @@
 const path = require("path");
 const childProcess = require('child_process');
+
 const copy = require("copy-webpack-plugin");
 const extract = require("mini-css-extract-plugin");
 const TerserJSPlugin = require('terser-webpack-plugin');
 const OptimizeCSSAssetsPlugin = require('optimize-css-assets-webpack-plugin');
-const webpack = require("webpack");
 const CompressionPlugin = require("compression-webpack-plugin");
+const CockpitPoPlugin = require("./src/lib/cockpit-po-plugin");
+
+const webpack = require("webpack");
 
 const nodedir = path.resolve((process.env.SRCDIR || __dirname), "node_modules");
 
@@ -20,7 +23,8 @@ const copy_files = [
 
 const plugins = [
     new copy({ patterns: copy_files }),
-    new extract({filename: "[name].css"})
+    new extract({filename: "[name].css"}),
+    new CockpitPoPlugin(),
 ];
 
 /* Only minimize when in production mode */