Not safe to close submission page if more uploads are queued

[ybnd]

Describe the bug
Recently, we added added a message to the submission page to inform users that it's safe to close the page when a Bitstream has been uploaded and is being processed on the REST side (e.g. being sent off to S3).
However, this message could prompt a user to close the page prematurely when there is an active upload queue, and subsequently canceling a number of uploads.

To Reproduce
Steps to reproduce the behavior:

1. Start a new submission
2. Drag a large number of files into the form to upload them
   • They will be added to a queue and uploaded one by one
   • The top of the page will say "it is now safe to close this page", but it isn't.

Expected behavior
We should only show the message when no more uploads are queued after the current one.

Related work
#1838

[github-actions]

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