Change Skill 'HABTM' CompetenceNodes association to 'belongs_to'
Added migration for Skill association changes
Adapted code to use the singular Skill.competence_node association
Fixed a couple of scopedcourse.code references to use scopedcourse.course_code
Changed a lot of scopedcourse.code references to .course_code
Fixed SQL queries that were still referring to scoped_courses
Fixed SQL query to 'competences' to point to 'competence_nodes'
Removed 'profile' association from Competence

Instead, CompetenceNode now has 'parent_competence' association to
itself, which models the same relationship. Thus, Competences can
be nested using the 'parent_competence' association.

Note: Profile model still exists in the code base and should refactored
away.