checkout process works for logged in user

Summary: Refs T1768: checkout process works for logged in user

Test Plan: log into the site either as admin or normal user, and place orders. Please try removing items from cart. Please try changing quantity. please try cart and item level promotions.

Reviewers: #everyone, gwarner

Reviewed By: #everyone, gwarner

Maniphest Tasks: T1768

Differential Revision: https://dev.blendtec.com/D1182