New invitation backend for portfolio invitations.

Portfolio invitations do not associate a user entity until the
invitation has been accepted. User info, including DOD ID, is held on
the invitation itself. When a user accepts and invitation, their user
entry is associated with the corresponding `portfolio_role` entry.

The same change will be applied to `application_role` and application
invitations. For now, small changes have been made to
application-related methods so that that flow works as-is.

tests/models/test_portfolio_role.py

@@ -65,7 +65,10 @@ def test_has_portfolio_status_history(session):
     #  to commit after create()
     PortfolioRoleFactory._meta.sqlalchemy_session_persistence = "flush"
     portfolio_role = PortfolioRoleFactory.create(portfolio=portfolio, user=user)
-    PortfolioRoles.enable(portfolio_role)
+    portfolio_role.status = PortfolioRoleStatus.ACTIVE
+    session.add(portfolio_role)
+    session.commit()
+
     changed_events = (
         session.query(AuditEvent)
         .filter(