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.
@dan-f Do we want "cohorted" to be translated? I'm confused about the usage of ugettext_noop (which I assume means don't translate) combined with the translation note.