Grade - Add an option to export the total in letters

Run the following behat tests on: @gradeexport_txt and @gradeexport_xml and make sure you don't get any error. Use a course that got some activities with graded items in it. Access the course. Go into Course Administration > Grades. Go into Export. For each of these types of export: Excel spreadsheet , Plain text file , XML File and OpenDocument spreadsheet . Click on the download button. Make sure the values are on Real display type. (Default) Select a single Grade export display type (Real, Percentage, Letter). Click on the download button. Make sure the values are on the format you selected. Repeat the testing instructions with multiple combination: (Real + Letter, Real + Percentage...) Make sure the exported file got the columns on the formats you selected and the titles have the type are on parenthesis (Real), (Percentage) or (Letter). Export grades as XML and CSV, import these and make sure the import procedure is working perfectly even with multiple display types on the exported files. (XML export is the only one without multiple display types)