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 we should probably use [`DefaultStorage`](https://docs.djangoproject.com/en/dev/ref/files/storage/#django.core.files.storage.DefaultStorage) here instead of `get_storage_class()()`. At least, that seems to be the "right way" according to the docs.