replaced group_default_instructor table with two separate join tables.
add migration for breaking up the group_default_instructor table.
bumped migration number.
bumped migration counter.
rewrote ALL the queries that were pulling data from group_default_instructor to use group_x_instructor/group_x_instructor_group as applicable.
replaced group_default_instructor with group_x_instructor_group in query.
replaced group_default_instructor with group_x_instuctor/group_x_instructor_group as applicable.
removed unused method deleteInstructorsForGroup().
replaced group_default_instructor with group_x_instructor_group.
refactored SQL query in getQueryResultsForInstructorsForGroup() to use new join tables instead of group_default_instructor.
refactored saveInstructorsForGroup() to store group/instructor and group/instructor_group associations in separate join tables instead of group_default_instructor.
Merge pull request #265 from stopfstedt/feat211

replaced group_default_instructor table with separate join tables.