Merge pull request #276 from stopfstedt/feat211_replace_ilm_session_facet_learner

replaced ilm_session_facet_learner with separate join tables.
aded migration for splitting up the ilm_session_facet_instructor table.
fixed insert triggers creation.
bumped migration number
bumped migration number.
updated code comments.
removed ilm_session_facet_instructor table definition and added new join tables in its place.
updated triggers according to schema changes.
removed redundant delete statement from nuke_session() stored procedure.
added drop/recreate copy_ilm_session_attributes_to_ilm_session() to migration.
updated implementation of stored proc. copy_ilm_session_attributes_to_ilm_session(). to make it work with new join tables.
search/replaced ilm_session_facet_instructor with ilm_session_facet_x_instructor/instructor_group as applicable.
updated table name in query.
updated getIndependentLearningFacet() to make it work with new join tables.
updated join table names.
reimplemented getInstructorsForSILM() according to recent schema changes.
renamed join table in queries according to recent schema changes.
Merge pull request #277 from stopfstedt/feat211_replace_ilm_session_facet_instructor

looks good!
Merge branch 'master' into mobilize
bugfix: return empty array under the "instuctors" key by default.