Assign students to cohorts by uploading a csv file

First draft of doc: course authors can use a csv upload to assign students to cohorts.
Will continue to update if additional info surfaces about UTF-8 and other requirements/limitations.

Suggestion: those `<DateTime>` fields are a little difficult to read. Although it's easy to determine the day, the time is difficult to read. You might want to point out that the list of file downloads is in reverse chronological order; that is, the most recently uploaded files are on top, and the oldest files are on the bottom.