Added in the ability to include a "calculated" total grade on the gradebook export, to facilitate the export of letter grades. Additionally, changed the GRADEBOOK_DISPLAY_WEIGHTED_TOTAL constant to only apply to the profile gradebook pages, and in the process of changing how the `settings` table works to allow organisation specific settings, as well as general settings without an organisation which can be overrided by those organisation specific settings, added the 'export_weighted_grade' setting, and the 'export_calculated_grade' settings to determine whether those should be output on the gradebook, and in the case of the calculated grade- what settings should be used.