* in_null for decimals defaults checks
* store letter settings only in database - no cfg settings
* separate configuration page for letters only - pluginselector+admin tree
* new capability to manage grade letters - similar to scales and outcomes
* added grade_letters db index
* grade letters related minor bugfixing
added new favoutcomes plugin settings page - to be used later, this replaces the grade_outcomes_grades table
admin settings - outcomes and scales both have settings page now