Improve import create behavior

For now, iin the bulk creation user interface, if you try to create a lot of files after applying some properties to all of them, it takes time. During this time:

• it would be nice to have a spinner
• it should not be possible to recreate on "Create".
• since anyway documents are not created in a transactional way (as they are created one by one), it should not be possible either to click on cancel button.
  ===> I suggest to have a sort of modal spinner

For now, it is possible to creaate all the documents n times.