Register TA's as user type "TA"

[Sho-Shoo]

Original user story: "As a TA, I want to be correctly registered as a TA rather than an instructor, so that students would know that a TA is answering rather than an instructor."

We ranked this user story as number 5 because although TA is not an established user type. TA's can still register as instructors. Although instructors (professors, course admins) may want to have more privileges in the system over TA's, the system will still work if this feature is not added.