Fewer db requests on bda tirage.

bda.algorithm
- use iterator to find max_groups, instead of a db request

bda.views.do_tirage
- select_related() are now focused on some relationships (they were
  taking useless relationships)
- bda-revente filling takes 1 request (each save and add was issuing
  1 request)

bda/algorithm.py

@@ -22,8 +22,7 @@ class Algorithm(object):
               show.requests
             - on crée des tables de demandes pour chaque personne, afin de
               pouvoir modifier les rankings"""
-        self.max_group = \
-            2 * choices.aggregate(Max('priority'))['priority__max']
+        self.max_group = 2*max(choice.priority for choice in choices)
         self.shows = []
         showdict = {}
         for show in shows: