Add properties to portfolio model

1. Funding duration

Returns a tuple of the earliest period of performance start date and
latest period of performance end date for all active task order in a  portfolio.

2. Days to funding expiration
Returns the numbei of days between today and the lastest
period performance end date of all active task orders

3. Active task orders
Returns a list of a portfolio's active task orders a

tests/models/test_portfolio.py

@@ -1,4 +1,12 @@
-from tests.factories import ApplicationFactory, PortfolioFactory
+from tests.factories import (
+    ApplicationFactory,
+    PortfolioFactory,
+    TaskOrderFactory,
+    CLINFactory,
+    random_future_date,
+    random_past_date,
+)
+import datetime
 
 
 def test_portfolio_applications_excludes_deleted():
@@ -7,3 +15,73 @@ def test_portfolio_applications_excludes_deleted():
     ApplicationFactory.create(portfolio=portfolio, deleted=True)
     assert len(portfolio.applications) == 1
     assert portfolio.applications[0].id == app.id
+
+
+def test_funding_duration(session):
+    # portfolio with active task orders
+    portfolio = PortfolioFactory()
+
+    funding_start_date = random_past_date()
+    funding_end_date = random_future_date(year_min=2)
+
+    TaskOrderFactory.create(
+        signed_at=random_past_date(),
+        portfolio=portfolio,
+        create_clins=[
+            {
+                "start_date": funding_start_date,
+                "end_date": random_future_date(year_max=1),
+            }
+        ],
+    )
+    TaskOrderFactory.create(
+        portfolio=portfolio,
+        signed_at=random_past_date(),
+        create_clins=[
+            {"start_date": datetime.datetime.now(), "end_date": funding_end_date,}
+        ],
+    )
+
+    assert portfolio.funding_duration == (funding_start_date, funding_end_date)
+
+    # empty portfolio
+    empty_portfolio = PortfolioFactory()
+    assert empty_portfolio.funding_duration == (None, None)
+
+
+def test_days_remaining(session):
+    # portfolio with task orders
+    funding_end_date = random_future_date(year_min=2)
+    portfolio = PortfolioFactory()
+    TaskOrderFactory.create(
+        portfolio=portfolio,
+        signed_at=random_past_date(),
+        create_clins=[{"end_date": funding_end_date}],
+    )
+
+    assert (
+        portfolio.days_to_funding_expiration
+        == (funding_end_date - datetime.date.today()).days
+    )
+
+    # empty portfolio
+    empty_portfolio = PortfolioFactory()
+    assert empty_portfolio.days_to_funding_expiration == 0
+
+
+def test_active_task_orders(session):
+    portfolio = PortfolioFactory()
+    TaskOrderFactory.create(
+        portfolio=portfolio,
+        signed_at=random_past_date(),
+        create_clins=[
+            {
+                "start_date": datetime.date(2019, 1, 1),
+                "end_date": datetime.date(2019, 10, 31),
+            }
+        ],
+    )
+    TaskOrderFactory.create(
+        portfolio=portfolio, signed_at=random_past_date(), clins=[CLINFactory.create()]
+    )
+    assert len(portfolio.active_task_orders) == 1