Show number of WCA competitions for competitor in Checklist

[fw42]

The old software had this feature where we showed the number of competitions for a competitor if it was a special number (like multiples of 10). See here.

Not sure how best to do this now that the WCA db is an external service and not sure if the API even supports that yet. Ideally, we would do some kind of batch request and not do one request for each competitor.

@timhabermaas, thoughts?

[fw42]

Also, should we do this on each request (will slow down rendering) or only once when the competitor is created (or rather: when the WCA ID is first entered) and then cache it in the db?