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 guess I thought it was easier to document what is expected, and at least "extensions" does seem more like a model property than a view (since it impacts the behavior of the uploader, at least on some platforms). "url" also seems like a model property rather than view.