Improving collection upload form type

Collection upload could now be processed asynchronously (no BC Break).

Developper still needs to handle all process on server side, but the component is now "ready to use" to make asynchronous upload (wich was not the case before this PR).
In a future PR, we'll improve CollectionUploadSubscriber to make it easier to use (for now, you have to make some custom code like handling asynchronous upload request and injecting asynchronous uploaded file to the form on server-side).