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.

I find "extra commas" confusing here. There aren't extra commas-- there are exactly the right number of columns (regardless of whether or not a cell is empty). Perhaps "every row must have the same number of commas, regardless of empty cells" or something like that? As it is, I would feel like I needed to put in extra commas when there is an empty cell.