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.
I feel that these tests would be easier to read if you dynamically constructed a stream from a string (or maybe a dict). Then you wouldn't need to check in all these files, and you'd be able to read the test without requiring a comment describing the contents of that file.

Something like:

    self._verify_csv_upload_acceptable_file(
        """
        instructor_user,,ManualCohort1
        ,student_user@example.com,AutoCohort1
        Ωπ,,ManualCohort1
        """
    )