MfSE Spring 2014 Implementation of User Stories add-teaching-assistant-to-course and copying-a-course-does-not-copy-tas

Based on requirements posted at http://whiteboard.sv.cmu.edu/pages/mfse_new_features.

1. add-teaching-assistant-to-course

As a course instructor, I want to add a TA to a course, so (s)he is listed as a TA on the main course page (under Course Tools) and can receive/send emails about the course. 
A TA can be any registered students (provided the student is not currently enrolled in that course). I may assign more than one TA to a course. Only course instructors or admins can assign a TA to a course. By default, anything that a course faculty can do, a TA can do too. (Anywhere the code does faculty.include?, it should also work for TAs.)

2. copying-a-course-does-not-copy-tas

When a course is copied, the TAs are not copied.