Don't restore 'Start Upload' button after an upload has been done

[qqmyers]

As of now, the dvwebloader can only be used once without being refreshed (could be fixed). If someone checks/unchecks some files in the interface, the 'Start Upload' button is restored and it is possible to try another upload. While the files go to S3, the step to add them to the dataset fails. At a minimum, the logic to turn on that button (whose purpose is to turn the button on/off depending on whether you have some files checked) should not restore it after an upload has been done.