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.
I'm not fond of the idea of having a helper method statically decide what the expected output should be in this case, since it depends on the input. I think the tests are already reasonably terse in that they just express the input and expected output.