[JENKINS-66395] Add postcss step to npm build.

Otherwise some display elements (e.g. select) will not be correctly rendered on all browsers.

Author: uhafner

etc/postcss.config.js

@@ -0,0 +1,10 @@
+module.exports = ctx => {
+    return {
+        plugins: {
+            autoprefixer: {
+                cascade: false
+            },
+            rtlcss: ctx.env === 'RTL' ? {} : false
+        }
+    }
+}