override ProtectedModel methods in Project model
settings_sample.py: add guardian setting to render the project's 403.html for permission denied

views.py: replace require_project_user decorator in view_project view with guardian decorator permission_required
views.py:
- view_project: add boolean variables for add & modify permission for user, passed to template to determine edit/add links

view_project.html:
- check for appropriate permissions before displaying add and edit links