Add multithreading to Dolos file download

[mikaelGusse]

Description

What?

Speed up report sending to Dolos by doing simple multithreading. This PR also improves the UX by providing a progress indicator once the user clicks the "View in Dolos" button alongside a description of Dolos and it's current status as a WIP tool.

Why?

Downloading submission files file by file is very slow when there are thousands of submissions. This speeds the process up substantially

How?

Using the concurrent.futures feature in python

Fixes #

Testing

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Translation

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Programming style

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• Reviewer has finished the code review
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