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.

@cahrens The "extra columns ignored" issue is covered later in that paragraph (see line 274-275). I need to rework that cell or rethink the table so that that information is easier to read. Table formatting is a known issue with our output that we have limited control over right now.