Bug resolved: Regarding modified_by field in models.py file (line #395)


**Functionalities implemented:**

1) *Only after successful creation/updation of history (version-file), the corresponding node will be saved/updated respectively.*
  - Files modified:
    1. models.py
       - Following functions updated:
         - create_or_replace_json_file() of HistoryManager class: MetaType added to collection_tuple
         - save() of Node class: try-catch blocks implemented

2) *In filldb.py file, newly created nodes of few of the TYPES was not saving modified_by and contributors. [NOTE: It only happens if you make new cloned copy and going for filldb command first time].*
  - This was the main reason for giving following error in line #395 (models.py file)... "User doesn't matching the Query"
  - This is now fixed.
  - Files modified:
    1. filldb.py
       - Code added to save modified_by and contributors wherever missing

**NOTE:**

1) If this error still comes, it means there are some nodes in database where either modified_by is 'None' or contributors is '[]'. In such case, please run following command:
```
  python manage.py filldb
```

2) And if, still, error persists.. manually check these fields' values and do necessary changes!