Upload CSV file for cohorting students

TNL-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.
If user is already in the cohort, a ValueError is thrown? If so, a comment would be helpful (are there other cases when a ValueError is thrown?).