small fixes

event/models.py

@@ -1,10 +1,12 @@
-from django.contrib.auth.models import User
+from django.contrib.auth import get_user_model
 from django.utils.translation import ugettext_lazy as _
 from django.core.validators import RegexValidator
 from django.core.exceptions import FieldError
 from django.db import models
 from communication.models import SubscriptionMixin
 
+User = get_user_model()
+
 
 class Event(SubscriptionMixin, models.Model):
     title = models.CharField(
@@ -164,6 +166,7 @@ class Activity(AbstractActivityTemplate):
     parent = models.ForeignKey(
                     ActivityTemplate,
                     related_name="children",
+                    blank=True,
                 )
     staff = models.ManyToManyField(
                     User,