group_admin field added to Group class


**Functionalities implemented**

1) group_admin field added under Group class
    
   - File modified:

   1) ```models.py```

       - ``` group_admin: [ObjectId] ```


   2) ```management/commands/add_fields_to_existing_documents.py```
      - Script also updated that adds this field (with [] as it's default value) in existing Group & Author documents
      
* Run this command: ```python manage.py add_fields_to_existing_documents```