explicitely convert is_accepted to boolean

The is_accepted property returns self.accepted_by or self.accepted_at.
When accepted_by is a User object (not None), it returns the User object
instead of a boolean.

The Django admin's boolean field renderer expects a boolean value
(True, False, or None), not a User object.

src/servala/core/models/organization.py

@@ -485,7 +485,7 @@ class OrganizationInvitation(ServalaModelMixin, models.Model):
     def is_accepted(self):
         # We check both accepted_by and accepted_at to avoid a deleted user
         # freeing up an invitation
-        return self.accepted_by or self.accepted_at
+        return bool(self.accepted_by or self.accepted_at)
 
     @property
     def can_be_accepted(self):