INT-4475: Outcome administration and mapping

* Added tool_outcome to install outcomes system and to take advantage of other plugin hooks.
* Added outcome as a core subsystem
* Added core_outcome to phpunit.xml.dist
* Added basic CRUD for outcome sets and their outcomes
* Added unit tests for most everything under outcome/classes/
* Added mapping of outcome sets to courses
* Added mapping of outcomes to questions
* Added mapping of outcomes to activities
* Added mapping of outcomes to rubrics
* Added new event: question_edited - fires on question edit
* Added new form element (MoodleQuickForm_map_outcome) to map outcomes to content
* Added new form element (MoodleQuickForm_map_outcome_set) to map outcome sets to a course