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/user.py

@@ -4,6 +4,7 @@ from sqlalchemy.dialects.postgresql import UUID
 
 from atst.models import Base, types, mixins
 from atst.models.permissions import Permissions
+from atst.models.portfolio_invitation import PortfolioInvitation
 
 
 users_permission_sets = Table(
@@ -31,6 +32,13 @@ class User(
         primaryjoin="and_(ApplicationRole.user_id==User.id, ApplicationRole.deleted==False)",
     )
 
+    portfolio_invitations = relationship(
+        "PortfolioInvitation", foreign_keys=PortfolioInvitation.user_id
+    )
+    sent_portfolio_invitations = relationship(
+        "PortfolioInvitation", foreign_keys=PortfolioInvitation.inviter_id
+    )
+
     email = Column(String)
     dod_id = Column(String, unique=True, nullable=False)
     first_name = Column(String)