ConsensusDebateResult causes N+1 queries in admin results page

[czlee]

On the main admin results page, this query occurs as many times as there are debates:

SELECT (1) AS "a"
  FROM "tournaments_round"
 WHERE ("tournaments_round"."tournament_id" = 1 AND "tournaments_round"."seq" > 1)
 LIMIT 1

Last few lines of stack trace:

/home/czlee/tabbycat/tabbycat/tournaments/models.py in debate_set_with_prefetches(519)
  populate_confirmed_ballots(debates, motions=True, results=True)
/home/czlee/tabbycat/tabbycat/results/prefetch.py in populate_confirmed_ballots(66)
  populate_results(confirmed_ballots)
/home/czlee/tabbycat/tabbycat/results/prefetch.py in populate_results(100)
  result.init_blank_buffer()
/home/czlee/tabbycat/tabbycat/results/result.py in init_blank_buffer(624)
  super().init_blank_buffer()
/home/czlee/tabbycat/tabbycat/results/result.py in init_blank_buffer(765)
  if len(self.sides) == 4 and self.debate.round.is_last:
/home/czlee/tabbycat/tabbycat/tournaments/models.py in is_last(579)
  return not self._rounds_in_same_sequence().filter(seq__gt=self.seq).order_by('seq').exists()

I think the check for whether there should be one or two winners in BP debates is causing it:

tabbycat/tabbycat/results/result.py

Lines 762 to 766 in 04541b0

	def init_blank_buffer(self):
	super().init_blank_buffer()
	self.scoresheet = self.scoresheet_class(positions=getattr(self, 'positions', None))
	if len(self.sides) == 4 and self.debate.round.is_last:
	self.scoresheet.number_winners = 1

The quick hackish way to result this would be to make Round.is_last a cached_property, but I didn't want to just do that in case there was some structurally better way to go about this, following the refactoring in #1180. We presumably want the information about the round to be passed cleanly to all the result objects or something, from populate_results() or maybe even from debate_set_with_prefetches(). (But I'm fine with just modifying Round.is_last if that's what should happen.)

(@tienne-B, thoughts?)