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.
`add_user_to_cohort` does an add, and possibly a remove to the CUG table. By wrapping calls to this method in a single transaction we ensure two things: If either of the add/remove fails, the other does not happen. That way nobody ends up in two cohorts. It also helps performance by reducing the number of database commits.