New file: decorators.py:
 - created custom view decorator @require_project_user for checking a user's membership to a project...interesting that I found a way to access the view parameters via the kwargs dictionary

models.py:
- new method is_project_user(project, user) for ProjectUserManager...used by the decorator above

views.py:
- import new decorator @require_project_user
- use @require_project_user on view_project view...tested and working!