Fix Dropzone following #15315 (#15353)

* Fix Dropzone following #15315

#15315 appears to have caused a change in the way Dropzone is imported - and it
now produces a module rather than the constructor.

This PR rather hackily just adds another Dropzone call to the result.

Signed-off-by: Andrew Thornton <art27@cantab.net>

* use destructured export

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: 6543 <6543@obermui.de>

web_src/js/features/dropzone.js

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