child slugs: require non-blank

Currently it is easy for a child with non-ascii name to get empty slug:
'Мааян' 'Паскин-Чернявский' => '-'
'Мааян' 'Паскин' => ''

Once any child has an empty slug, many pages break with NoReverseMatch
errors.  This commit at least prevents setting such names;
slug computation to be fixed later.

core/models.py

@@ -80,6 +80,7 @@ class Child(models.Model):
         verbose_name=_('Birth date')
     )
     slug = models.SlugField(
+        blank=False,
         editable=False,
         max_length=100,
         unique=True,