WL-98 As a course instructor, auto-register (as well as auto enroll) a list of students into my course

@chrisndodge here are the changes related to WL-98 for your quick reivew. 
we have written the test cases but only one coverage is missing in our code.  
currently we did not check/test the code for quality.

In order to test this feature you need to set the flag: 
FEATURES['ALLOW_AUTOMATED_SIGNEDUPS'] = True
Typo: "te email address only". Please clarify that the user will be enrolled via the email address. Also, instead of saying "change of username" (which makes it sound like you will change the username associated with the email address", say "different username"