Documents: Add default filtering of stubs

Add filter(is_stub) to the default Document model manager.

Now only the Passthrough manager can access document stubs.

Remove the explicit filtering of stubs from code that obtains
the queryset from the default document manager.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/documents/dashboard_widgets.py

@@ -55,7 +55,7 @@ class DashboardWidgetDocumentsTotal(DashboardWidgetNumeric):
         )
         self.count = AccessControlList.objects.filter_by_access(
             permission=permission_document_view, user=request.user,
-            queryset=Document.objects.filter(is_stub=False)
+            queryset=Document.objects.all()
         ).count()
         return super(DashboardWidgetDocumentsTotal, self).render(request)