Tweak language surrounding grade download button

@ormsbee Can you take a look at this? This is wording Mark (Rudnick) & I worked on to hopefully better explain things for when we let all users generate reports.

@jinpa Since I was in this file already, I changed the wording a little bit around the list of files to hopefully prevent the confusion you had about the "5 minute link expiration" issue. Can you take a look and see if this is clearer?