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.
So with this approach, you don't have to delete the temp file? Should you use approach in your other test (where you do make sure to delete the created files)?