Ignore deleted entrants when recording races (Option 2 of 2)

[TCKLZ]

Summary

• Allow race recording even when one or more entrants have deleted their account.
• Exclude deleted entrants from the rating calculation so rate_race() does not dereference a null user.
• Return early if no remaining entrants can be rated.

Testing

• python manage.py check
• python manage.py test racetime (0 tests found)
• python -m py_compile racetime/models/race.py racetime/rating.py

Note
This changes scoring behavior: deleted entrants are ignored for rating purposes, so points are calculated from the remaining users.

Commit 2; Updated to keep the deleted-user guard for team races, since ignoring a deleted entrant there could imbalance teams and skew ratings.

Also added a check so non-team races are only recorded if at least two entrants with accounts remain after deleted users are ignored.

[deains]

Thanks for your contribution! We do need to account for a couple of extra cases to make this work though.

1. If the race is a team race, the guard should remain in place. Recording a team race with one or more deleted users will lead to imbalanced teams and that will skew the points awfully.
2. If the race has only one entrant left after accounting for deleted users, it should not be recorded. A race with a single entrant cannot be meaningfully scored.