added new entry to language file.
added proper docblock to action saveGroupModelTree(). updated user input pre-processing to better handle JSON decoding issues. removed obsolete code docs.
improved method name.
added getter methods for retrieving instructor/instructor-group associations with a group, as well as retrieving these associated entities themselves. added sane methods for updating these associations. general tidying up and additions of code docs.
optimized the updating process of instructor/instructor-group associations with learner groups. refs #3088
added logic to only add an "update group" entry to the audit trail if the db record was actually updated. refs #3088
added query result cleanup.