Image upload in permanent state of "uploading"

[olvado]

There are times when an image upload appears to fail without throwing any errors - accordingly the spinner remains on top of the image and the submit button is disabled, continuing to say "Please wait".

It's possible that the most common cause of this behaviour is when quickly adding multiple image blocks without waiting for each image to upload before attempting the next.

This also highlights the need for a cancel upload option when removing an image block before its upload is completed.

[raffij]

Let's remove the fileUploads from the global count when the block is removed. Something that this doesn't do is cancel the fetch request. Have to see if there's a way to fix that. Otherwise uploads continue, but don't fire the callback.