Upload CSV file for cohorting students

STUD-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.
This test seems a little underpowered. It doesn't test that you can upload a CSV. It just tests that there's a piece of text with a given label. It would be great to actually press the button and verify that the correct request is sent to the server. I'd also like to verify that the correct message is then shown about checking the data downloads tab.

I can see that the FileUploaderView already tests most of the error conditions so I'm okay not repeating all of those, but I think it would be good to verify the normal behavior.