Bug resolved: Faulty values of modified_by and contributors fields in existing documents

**Files modified:**

1) management/commands/filldb.py
   - If modified_by and contributors is found None and empty respectively, then add creator of that resource as their values.
   - Updates faulty values of modified_by and contributors fields

2) Following views-files updated with code (add/modify modified_by and/or contributors field):
   - home.py
   - ajax_views.py
   - file.py
   - forum.py
   - notify.py
   - page.py
   
3) Following views-files updated with code (Tags are added only if provided some value): 
   - methods.py
   - quiz.py


**NOTES:**

1) Please run following command:
   ```
      python manage.py filldb
   ```