User: make username case insensitive (and save original styling).

We want 'User' and 'UsEr' to mean the same user. Apparently that's not the default in
django. This normalizes the user to ensure we enforce this.

django_etebase/serializers.py

@@ -382,7 +382,12 @@ class AuthenticationSignupSerializer(serializers.Serializer):
         user_data = validated_data.pop('user')
 
         with transaction.atomic():
-            instance, _ = User.objects.get_or_create(**user_data)
+            try:
+                instance = User.objects.get_by_natural_key(user_data['username'])
+            except User.DoesNotExist:
+                # Create the user and save the casing the user chose as the first name
+                instance = User.objects.create_user(**user_data, first_name=user_data['username'])
+
             if hasattr(instance, 'userinfo'):
                 raise serializers.ValidationError('User already exists')