Upload CSV file for cohorting students

STUD-735

@andy-armstrong You can start reviewing the front-end changes and tests for this PR. I'll send you an email with some information about the sandbox.

At this point, you can also edit files created in Excel, unless if they have unicode characters in them.
Agreed, but I don't want to change the logic of the code where this was pulled from (lms/djangoapps/django_comment_client/base/views.py). That code does not seem to have automated tests, and although I have stepped through the code in the debugger to make sure I am not changing any logic, I haven't been able to completely test the functionality on devstack or sandbox. To reduce risk, I'd like to keep the exception handling the same.