(WIP) 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
Also, per some of @cahrens' comments, there's some light existing UI to support inline confirmation/error messages to users (added during the first cohort management iteration). You'd have canned, already styled UI if you leveraged that (which requires some change in the markup here - the cohorts templates would point you in the right direction).