Membership training statistics page - seat count should not include cancellations

[elichad]

Related Slack conversation: https://carpentries.slack.com/archives/GDC238XEJ/p1699256079140289

In the emails we send to members, the listed number of seats used can be different to the number that appears on the /reports/membership_trainings_stats/ page. There are a few reasons for this (see the Slack conversation above) but one of them is that when a trainee cancels their attendance (thus freeing their seat), the cancellation is tracked in a TrainingProgress, but not on the associated Task. I think the report just counts Tasks, and so counts cancelled seats when it shouldn't.