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.
I think just having that example ("Progress: {action} {succeeded} of {attempted} so far") would be very helpful as it gives context. Having said that, I wonder whether there are languages where these different component parts can't be so easily concatenated together. I wouldn't worry about that for now though.