MDL-10386

MDL-10386
- Renamed grade_report_shownotes preference to grade_report_showfeedback
- Added grade_report_quickgrading and grade_report_quickfeedback preferences
- Re-organised the grade_get_icons() function: The icons are all prepared first,
    then the preferences and conditions are applied to the selection of icons.
    This allows for a third $icons parameter, an array of icon names, which
    explicitly sets which icons should be returned by the function, regardless
    of preferences and options.
- Identified and fixed MDL-10391 bug with updating boolean user preferences (advcheckbox used)