Migrated type_of field from unicode to ObjectId

This modification is required to store the ObjectId of GSystemType, which shows the node is type_of particular GSystemType

Modification in : 
```
filldb.py  -->  Created GSystemType 'profile_pic' 
models.py  --> updated 'type_of' field in Node collection as ObjectId
userDashboard.html  --> modification according to 'profile_pic' ObjectId
ndf_tags.py  -->  modification according to 'profile_pic' ObjectId in 'get_profile_pic()'
file.py  -->  Replaced type_of object with an ObjectId of GSystemType 'profile_pic'
userDashboard.py  -->  modification according to 'profile_pic' ObjectId
migrate_type_of.py  -->  This script will migrate the already consisting type_of field in documents as unicode value to ObjectId of GSYstemType 'profile_pic'
```
Previously 'profile_pic'  value is hard coded to find the type of uploaded profile image to distinguish between profile image and other uploaded images in metastudio, Now GSystemType 'profile_pic' created and its ObjectId stores in 'type_of' field  

[NOTE : Please run the script after latest pull as "python manage.py migrate_type_of" ]