Add letter in pool display

Signed-off-by: Emmy D'Anello <emmy.danello@animath.fr>

participation/models.py

@@ -359,6 +359,16 @@ class Pool(models.Model):
         ]
     )
 
+    letter = models.PositiveSmallIntegerField(
+        choices=[
+            (1, 'A'),
+            (2, 'B'),
+            (3, 'C'),
+            (4, 'D'),
+        ],
+        verbose_name=_('letter'),
+    )
+
     participations = models.ManyToManyField(
         Participation,
         related_name="pools",
@@ -406,6 +416,7 @@ class Pool(models.Model):
     class Meta:
         verbose_name = _("pool")
         verbose_name_plural = _("pools")
+        ordering = ('round', 'letter',)
 
 
 class Passage(models.Model):