Make portfolio invitation specific to portfolio

- add a base domain class
- extract shared model code to mixin
- rename invitation classes
- invitation model relationship to portfolio_role name is now more
  generic "role"

tests/routes/portfolios/test_members.py

@@ -45,7 +45,7 @@ def test_create_member(client, user_session):
     assert response.status_code == 200
     assert user.full_name in response.data.decode()
     assert user.has_portfolios
-    assert user.invitations
+    assert user.portfolio_invitations
     assert len(queue.get_queue()) == queue_length + 1
     portfolio_role = user.portfolio_roles[0]
     assert len(portfolio_role.permission_sets) == 5