MDL-40053 events: Generator for core_notes
MDL-40053 events: write events for core_notes

Following events were added in this commit :-
core\event\note_created
core\event\note_deleted
core\event\note_updated
core_event\notes_viewed
MDL-40053 events: Write unit tests for note events
MDL-40053 events: Replace add_to_log() with new events in core_notes