Minor fix - Group listing and User-access to private groups and their resources


**Functionalities implemented:**

**1) Group listing updated (Both in drop-down as well as in group's list-view)**

   - Additional check of group_admin field is added to necessary queries
     - This makes group visible to admin users in required places mentioned above
   
   - File modified:
   
     1) ```views/group.py``` - group() updated
     2) ```templatetags/ndf_tags.py``` - get_group_type() updated
     
**2) User-access to private groups and their resources**

   - Restricting unauthorized access using urls
     - Unless/until user belongs to a private group (as a admin-user or creator or subscribed-user), he/she can't access that group or it's resources even if they know the corresponding url!
     
   - File modified:
   
     1) ```templatetags/ndf_tags.py``` - get_group_type() updated

        - Extracting group's ObjectId and checking passed user's id exists in it's required field's (group_admin, author_set, created_by)
        - In addition to above, query also checks whether group is public or not
        - If any of the above check is satisfied, access is granted to the user. Otherwise, error is thrown!