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.
We actually do display the errors. This can include errors like the file has the wrong extension, the file does not have the correct columns, the S3 bucket isn't properly configured, etc. They are all much, much more useful errors than a generic one.