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 know that we display the errors, but it doesn't seem a good idea to catch all exceptions and display them as is to the user. Most if not all of them won't be localized, and some of them could reflect all sorts of internal issues that are not relevant.