Make biography optional

contour/models.py

@@ -75,7 +75,7 @@ class Participant(models.Model):
   projects = models.ManyToManyField('Project', through='ProjectParticipant')
   name_nl = models.CharField(max_length=255, null=True, blank=True)
   sort_name_nl = models.CharField(max_length=255, null=True, blank=True)
-  bio_nl = models.TextField(verbose_name="Biography")
+  bio_nl = models.TextField(verbose_name="Biography", null=True, blank=True)
   name_en = models.CharField(max_length=255, verbose_name="Name (English)", null=True, blank=True)
   sort_name_en = models.CharField(max_length=255, verbose_name="Sortname (English)", null=True, blank=True)
   bio_en = models.TextField(verbose_name="Biography (English)", null=True, blank=True)