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 think a default buffer_size should be provided. I don't see why different callers would want different values, and it just adds burden on the caller to decide what an appropriate buffer_size is.