[WIP] added dao and rest endpoint for retrieving invited users

by service and by id.

app/dao/invited_user_dao.py

@@ -1,6 +1,16 @@
 from app import db
 
+from app.models import InvitedUser
+
 
 def save_invited_user(invited_user):
     db.session.add(invited_user)
     db.session.commit()
+
+
+def get_invited_user(service_id, invited_user_id):
+    return InvitedUser.query.filter_by(service_id=service_id, id=invited_user_id).one()
+
+
+def get_invited_users_for_service(service_id):
+    return InvitedUser.query.filter_by(service_id=service_id).all()