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/models/test_portfolio_invitations.py

@@ -0,0 +1,45 @@
+import datetime
+
+from atst.models import InvitationStatus, PortfolioRoleStatus
+
+from tests.factories import (
+    PortfolioInvitationFactory,
+    PortfolioFactory,
+    UserFactory,
+    PortfolioRoleFactory,
+)
+
+
+def test_expired_invite_is_not_revokable():
+    portfolio = PortfolioFactory.create()
+    user = UserFactory.create()
+    portfolio_role = PortfolioRoleFactory.create(
+        portfolio=portfolio, user=user, status=PortfolioRoleStatus.PENDING
+    )
+    invite = PortfolioInvitationFactory.create(
+        expiration_time=datetime.datetime.now() - datetime.timedelta(minutes=60),
+        role=portfolio_role,
+    )
+    assert not invite.is_revokable
+
+
+def test_unexpired_invite_is_revokable():
+    portfolio = PortfolioFactory.create()
+    user = UserFactory.create()
+    portfolio_role = PortfolioRoleFactory.create(
+        portfolio=portfolio, user=user, status=PortfolioRoleStatus.PENDING
+    )
+    invite = PortfolioInvitationFactory.create(role=portfolio_role)
+    assert invite.is_revokable
+
+
+def test_invite_is_not_revokable_if_invite_is_not_pending():
+    portfolio = PortfolioFactory.create()
+    user = UserFactory.create()
+    portfolio_role = PortfolioRoleFactory.create(
+        portfolio=portfolio, user=user, status=PortfolioRoleStatus.PENDING
+    )
+    invite = PortfolioInvitationFactory.create(
+        role=portfolio_role, status=InvitationStatus.ACCEPTED
+    )
+    assert not invite.is_revokable