BUG: Datasets disappear from the status page before they are completed`

[Rosencrantz]

Describe the bug
When uploading a large dataset with large raw documents that produce small numbers of tasks to be completed there is a high chance that there will be a period of time when a dataset has no pending tasks (after existing tasks have finished and no new tasks have started). When a dataset has no pending tasks it is treated as completed and is removed from the status page. This resets all the metrics. After the next document has been uploaded the dataset is added to the status page again, but it looks as though this in a new 'job'.

Expected behavior
Ideally when a dataset is in progress it remains on the status page until all the documents have been uploaded. It may be enough to add a delay to deal with 80% of these issues when a document is ready shortly (less than a minute?) after the previous set of tasks has completed

Aleph version
All