is_gstaff() function added under Group class

**Functionalities implemented:**

1) ```is_gstaff()``` function added under *Group* class

   - Checks whether given user belongs to GStaff (i.e. super-user, creator or admin-user of the group) or not.
   
2) Assignment tags added/modified in ```templatetags/ndf_tags.py```:

   - Added: ```check_is_gstaff()``` which in turn calls is_gstaff()
   - Modified: *check_group_admin()* replaced by ```check_is_gstaff_for_gapp()```
   
3) Call to *user_access_policy* replaced with ```check_is_gstaff()``` so that specific html-elements are only visible to GStaff people

   - Files modified:
   
     1) templates/ndf/mis_base.html
     2) templates/ndf/mis_base_list.html
     3) templates/ndf/event_details.html
     3) templates/ndf/person_details.html

4) Call to *check_group_admin* replaced with ```check_is_gstaff_for_gapp()``` so that specific GAPPS are only visible to GStaff people

   - File modified:
   
     1) templates/ndf/gapps_menubar.html