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.

atst/domain/portfolio_roles.py

@@ -163,8 +163,9 @@ class PortfolioRoles(object):
         return portfolio_role
 
     @classmethod
-    def enable(cls, portfolio_role):
+    def enable(cls, portfolio_role, user):
         portfolio_role.status = PortfolioRoleStatus.ACTIVE
+        portfolio_role.user = user
 
         db.session.add(portfolio_role)
         db.session.commit()