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.

src/lib/cockpit-po-plugin.js

@@ -0,0 +1,74 @@
+const path = require("path");
+const glob = require("glob");
+const po2json = require('po2json');
+const Jed = require('jed');
+
+module.exports = class {
+    apply(compiler) {
+        compiler.hooks.emit.tapPromise(
+            'CockpitPoPlugin',
+            compilation => Promise.all(glob.sync('po/*.po').map(f => this.buildFile(f, compilation)))
+        );
+    }
+
+    prepareHeader(header) {
+        if (!header)
+            return null;
+
+        var body, statement, ret = null;
+        const plurals = header["plural-forms"];
+
+        if (plurals) {
+            try {
+                /* Check that the plural forms isn't being sneaky since we build a function here */
+                Jed.PF.parse(plurals);
+            } catch(ex) {
+                fatal("bad plural forms: " + ex.message, 1);
+            }
+
+            /* A function for the front end */
+            statement = header["plural-forms"];
+            if (statement[statement.length - 1] != ';')
+                statement += ';';
+            ret = 'function(n) {\nvar nplurals, plural;\n' + statement + '\nreturn plural;\n}';
+
+            /* Added back in later */
+            delete header["plural-forms"];
+        }
+
+        /* We don't need to be transferring this */
+        delete header["project-id-version"];
+        delete header["report-msgid-bugs-to"];
+        delete header["pot-creation-date"];
+        delete header["po-revision-date"];
+        delete header["last-translator"];
+        delete header["language-team"];
+        delete header["mime-version"];
+        delete header["content-type"];
+        delete header["content-transfer-encoding"];
+
+        return ret;
+    }
+
+    buildFile(po_file, compilation) {
+        return new Promise((resolve, reject) => {
+            const jsonData = po2json.parseFileSync(po_file);
+            const plurals = this.prepareHeader(jsonData[""]);
+
+            let output = JSON.stringify(jsonData, null, 1);
+
+            // We know the brace in is the location to insert our function
+            if (plurals) {
+                pos = output.indexOf('{', 1);
+                output = output.substr(0, pos + 1) + "'plural-forms':" + String(plurals) + "," + output.substr(pos + 1);
+            }
+
+            // wrap JSON output into cockpit.locale() call
+            output = 'cockpit.locale(' + output + ');\n';
+
+            const lang = path.basename(po_file).slice(0, -3)
+            compilation.assets['po.' + lang + '.js'] = { source: () => output, size: () => output.length };
+            resolve();
+        });
+    };
+};