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.
This test point and the 3 below it have a log of duplication. You can create a helper method that does all of this--

self.client.login(username=self.staff_user.username, password='test')
response = self.call_add_users_to_cohorts(file_contents)
self.assertEqual(response.status_code, 400)
result = json.loads(response.content)
self.assertEqual(result['error'], expected_error)