Grade letters currently work like this:

    Site preferences are default for:
    Course user preferences which affect : display for graders only

The problem is that the grade letters need to work in lots of reports/imports/exports and they need to work consistently, like this:

    1. Site settings define overall letter boundaries which act as a default.
    2. Course settings are set on plugin menu under Edit -> Grade Display (each can be site default or some local override). Also here is a course default for "Real / Percentage / Letters / Default"
    3. Grade_item settings have another setting for "Real / Percentage / Letters / Default"
    
1 would require a new grade_letters table (set courseid to zero)
2 would use the same table (set courseid to courseid) for the boundaries, and a config_plugins entry for the grade display
3) would require a new field on grade_item called "display".

There needs to be changes in the grade classes too so that these are all resolved correctly.