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/domain/test_portfolio_roles.py

@@ -7,7 +7,7 @@ from atst.models.portfolio_role import Status as PortfolioRoleStatus
 from tests.factories import (
     PortfolioFactory,
     UserFactory,
-    InvitationFactory,
+    PortfolioInvitationFactory,
     PortfolioRoleFactory,
 )