GroupUser table not properly deleting rows

[Rlin232]

1. When user is deleted (from admin panel - Users table), we need to remove the user from the GroupUser table
2. When group is deleted (from admin panel - Groups table), we need to remove all users from the GroupUser table

Currently, deletion of external keys only results in a "null" value being changed for the userId in the row. We need to implement cascade deletes to be triggered when the user and group is delete.