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 don't see any reason why we shouldn't check in get_random_post_override if it is useful for generating data. I believe Andy is just objecting to "Rule the world".