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"

atst/models/portfolio_invitation.py

@@ -0,0 +1,41 @@
+from sqlalchemy import Column, ForeignKey
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import relationship, backref
+
+from atst.models import Base
+from atst.models.mixins import TimestampsMixin, AuditableMixin, InvitesMixin
+
+
+class PortfolioInvitation(Base, TimestampsMixin, AuditableMixin, InvitesMixin):
+    __tablename__ = "invitations"
+
+    portfolio_role_id = Column(
+        UUID(as_uuid=True), ForeignKey("portfolio_roles.id"), index=True
+    )
+    role = relationship(
+        "PortfolioRole",
+        backref=backref("invitations", order_by="PortfolioInvitation.time_created"),
+    )
+
+    @property
+    def portfolio(self):
+        if self.role:  # pragma: no branch
+            return self.role.portfolio
+
+    @property
+    def portfolio_id(self):
+        return self.role.portfolio_id
+
+    @property
+    def event_details(self):
+        return {"email": self.email, "dod_id": self.user_dod_id}
+
+    @property
+    def history(self):
+        changes = self.get_changes()
+        change_set = {}
+
+        if "status" in changes:
+            change_set["status"] = [s.name for s in changes["status"]]
+
+        return change_set