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

user/models/user.py

@@ -60,6 +60,29 @@ class User(AbstractUser):
             name=ETS_GROUP
         ).exists()
 
+    def is_default_group_only(self) -> bool:
+        """ Checks if the user is only part of the default group
+
+        Args:
+
+        Returns:
+            bool
+        """
+        return not self.in_group(ZB_GROUP) and not self.in_group(ETS_GROUP)
+
+    def in_group(self, group: str) -> bool:
+        """ Checks if the user is part of a group
+
+        Args:
+            group (str): The group's name
+
+        Returns:
+            bool
+        """
+        return self.groups.filter(
+            name=group
+        )
+
     def send_mail_shared_access_removed(self, obj_identifier, obj_title, municipals_names):
         """ Sends a mail to the user in case of removed shared access