Web UI: add status "ongoing" for "save code now" requests

The "Browse save requests" panel shows various statuses for submitted save code now requests, and that great. The gap between the last two—"scheduled" and then "succeeded"—is a bit too large and can be very puzzling for requests that takes a lot to complete. It would be nice to have an intermediate "ongoing" status while the loader serving the given request is running. (I've no idea if swh-web has access to the needed info, so this might need cascading to other components…)

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Migrated from T2614 (view on Phabricator)