New attribute ('property_order') added to GSystemType class


**Functionalities implemented:**

1) Attribute added in GSystemType class - property_order (data-tye: list)
   - This field is added so as to hold the order in which user-defined attributes should be displayed in templates

**Files modified:**

1) models.py
   - Following attribute added in GSystemType class:
     ```
     'property_order': []
     ```
     
2) add_fields_to_existing_documents.py
   - Appends existing GSystemType documents with this field (default value as [])