MDL-37834 assign: avoid newlines in status column of csv

Previously we were generating a CSV file with newlines in it, which
seems to cause problems importing into some versions of MS Excel.

Instead seperate status lines by '-' - this column is only used to
provide context to the teacher in the spreadsheet.

mod/assign/gradingtable.php

@@ -972,7 +972,7 @@ class assign_grading_table extends table_sql implements renderable {
         }
 
         if ($this->is_downloading()) {
-            $o = strip_tags(str_replace('</div>', "\n", $o));
+            $o = strip_tags(rtrim(str_replace('</div>', ' - ', $o), '- '));
         }
 
         return $o;