address "lag" in transfer state change when complete button is pushed.

with our polling, it's possible that, because of lag time/delay in response, the current situation where we "disable" the complete button when the state changes to "pending" (files are being created) could result in a user clicking the complete button multiple times, even though, technically, the process has started.

Initial work is to "disable" the button once clicked. This addresses a single user in their own browser, but does not address the ability for other users (or the same user in a different browser) from also "clicking" the button.

So, further investigation by Salem Aouana indicates that we might be better off to go ahead and tie the transition directly to the button click (and not wait for the response from the server).