upload-modal: rename Cancel to Close
upload-modal: rename Cancel to Close

This is really a close button. We will soon be keeping the modal open
after the files have been uploaded (to show the status of each upload)
and this button will then be used to close the window.

mgeisler/swift-browser,zerovm/swift-browser,zerovm/swift-browser,mindware/swift-browser,mindware/swift-browser,mgeisler/swift-browser