Unit test user app

* adds unit test for User model and forms
* refactors functions from user_checks.py into User class and drops user_checks.py

konova/models/object.py

@@ -640,12 +640,11 @@ class ShareableObjectMixin(models.Model):
         Returns:
 
         """
-        from konova.utils.user_checks import is_default_group_only
         users = self.shared_users
         cleaned_users = []
         default_users = []
         for user in users:
-            if not is_default_group_only(user):
+            if not user.is_default_group_only():
                 cleaned_users.append(user)
             else:
                 default_users.append(user)