Feature collection upload async

Handle asynchronous file upload for collection_upload form type.

This is ready to merge.

**TODO**
 - [ ] Test and review: @loostro, if you want to give me some feedbacks, don't hesitate
 - [x] Add documentation


Please, do not hesitate to comment

(https://github.com/sescandell/CollectionUploadSample is up to date with this functionnality).