Move calc of a portfolio's obligated funds to prop

Add a property on the portfolio model to calculate the total
obligated funds for a portfolio. This replaces a one-off calculation
in a view function, and sets up functionality for future access

atst/models/portfolio.py

@@ -89,6 +89,12 @@ class Portfolio(
     def active_task_orders(self):
         return [task_order for task_order in self.task_orders if task_order.is_active]
 
+    @property
+    def total_obligated_funds(self):
+        return sum(
+            (task_order.total_obligated_funds for task_order in self.active_task_orders)
+        )
+
     @property
     def funding_duration(self):
         """