Upload CSV file for cohorting students

TNL-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 believe ``.submit-file-button`` is used for form submission/system logic. ``action`` and ``action-[PURPOSE]`` are UI-focused classes we generally tend to add.  ``is-disabled`` is needed since we only enable the button once a file has been provided in this UI. I'll remove ``action-primary`` with that in mind.