Move dropzone.js to npm/webpack (#10645)

- unvendor dropzone and upgrade it from 4.2.0 to 5.7.0
- make `csrf` available on window.config

web_src/js/features/dropzone.js

@@ -0,0 +1,9 @@
+export default async function createDropzone(el, opts) {
+  const [{ default: Dropzone }] = await Promise.all([
+    import(/* webpackChunkName: "dropzone" */'dropzone'),
+    import(/* webpackChunkName: "dropzone" */'dropzone/dist/dropzone.css'),
+  ]);
+
+  Dropzone.autoDiscover = false;
+  return new Dropzone(el, opts);
+}