add cancel download / escape button for downloads

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Problem: users are very likely going to attempt very large downloads and realise they don't have the patience for it (especially for continuous downloads). We need an button they can press that effectively Ctrl-C's out of it (currently the download remains running in the background).

The only hiccup is that the database should also be cleaned up based on their progress thus far. So, the button press should also run db_manager.join_continuous_segments(settings.proccess.gap_tolerance) immediately afterwards.

I may have mis-spoke in the meeting just now; the database IS updated with each archive_request call, so it will be updated along the way. We just need to clean it up at the end regardless if the download is terminated normally or prematurely.