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 having a default size is useful, though this 10MB size may not be.... Perhaps the default size should be whatever you and Ed come up with? I don't feel terribly strongly about it, but it does seem like it would be helpful for others in the future to have an idea of what a reasonable max size is.