Models file updated: Replaced field-name modified_by by contributors ([int]) and new modified_by (int) field added

**Functionalities implemented:**

1. In models file, following fields added/modified:

   1) Field modified: *location*
      - This field's data-type changed from 'dict' to '[dict]'
   
   2) Field modified: *modified_by*
      - This field's data-type changed from '[int]' to int
      - Because as per discussion, this field is now onwards going to store detail about the user (primary key from django's User database) who lastly modified the document.
      
   3) Field added: **contributors**
      - This field's data-type is going to be '[int]'
      - This field holds the values that previously modified_by field was holding!


**Files created/modified:**

1. File created: **update_fields_in_existing_documents.py**
   ```
   NOTE: Now onwards use this file for updating fields in existing documents; no need to create new files for migrating/updating any database fields.
   ```
   1) Updates location fields' values from '{}' (empty dictionary) to '[]' (empty list)
   2) Renames field: 'modified_by' to 'contributors'
   
2. File modified: *add_fields_to_existing_documents.py*
   1) Adds "modified_by" field with None as it's default value

3. File modified: models.py
   1) Renamed 'modified_by' to 'contributors'
   2) Added new modified_by field (data-type: int)
   3) user_details_dict property modified: Changed key from modified_by to contributors and added new key as modified_by
   4) **While updating history (version-file), now it stores username of the specific user who modifies the document.**

4. File modified: views/methods.py
   1) Code added to save modified_by and contributors.
   
5. File modified: views/ajax_views.py
   1) Modifications regarding resolving following error: Can't concatenate 'int' to 'str'...
   
6. In Following templates, Replaced modified_by by contributors: 
   - templates/ndf/node_ajax_view.html
   - templates/ndf/node_details_base.html
   - templates/ndf/page_details.html
   - templates/ndf/page_list.html
   - templates/ndf/quiz_item_details.html
   - templates/ndf/userDashboard.html
 
6. In Following views, Code added/modified to save modified_by and contributors fields accordingly: 
   - views/ajax_views.py
   - views/group.py
   - views/methods.py
   - views/quiz.py
   - views/userDashboard.py


**NOTES/Steps to be performed:**

```
  NOTE - Please run following commands in given order:
        1) python manage.py update_fields_in_existing_documents
        2) python manage.py add_fields_to_existing_documents
```

```
  NOTE: Now onwards use update_fields_in_existing_documents.py file for updating fields in existing documents; no need to create new files for migrating/updating any database fields.
```