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.
Thanks for pointing this out. Changing the default to check order.

Order is irrelevant for the cohort results csv, since it's built up from an unordered dictionary.