Export stuck for multiple testers' accounts on QA

[imnasnainaec]

Reported on 2024 July 26: https://jira.sil.org/browse/LT-21842

It occurred for at least two people and persisted through restarting the browser.

Follow-up detail from Aug 1:
After "clearing all the browser's data for https://qa-kube.thecombine.app[...], the export button becomes enabled, but if we try to export, it starts loading again."

Update from Aug 7: They are able to export again. Whatever the original issue trigger was, we still need to enable the user to cancel/clear stalled exports.

Action: Add "cancel export" button next to in-progress/loading export button (either a button with text or an X button with hover text, whichever looks better).