Route for adding new application member

- domain method for creating a new application member
- ApplicationInvitations domain class
- nested form for adding a new user that holds user data, application
  permission sets, and environment roles
- Invitation service can infer invitation type based on role it's given
- new invitation email templates

atst/models/user.py

@@ -5,6 +5,7 @@ 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
+from atst.models.application_invitation import ApplicationInvitation
 
 
 users_permission_sets = Table(
@@ -39,6 +40,13 @@ class User(
         "PortfolioInvitation", foreign_keys=PortfolioInvitation.inviter_id
     )
 
+    application_invitations = relationship(
+        "ApplicationInvitation", foreign_keys=ApplicationInvitation.user_id
+    )
+    sent_application_invitations = relationship(
+        "ApplicationInvitation", foreign_keys=ApplicationInvitation.inviter_id
+    )
+
     email = Column(String)
     dod_id = Column(String, unique=True, nullable=False)
     first_name = Column(String)