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.
Nit: the "once more" here is confusing. This is the first time we are iterating...

I do wonder if it is necessary to iterate through twice.