refactor to use moodle_core abstraction layer for context_course #319

Job	Run time
define-matrix	4s
test (8.3, main, pgsql, true)	3m 25s
test (8.3, main, mariadb, true)	3m 27s
test (8.1, MOODLE_404_STABLE, pgsql, false)	3m 18s
test (8.1, MOODLE_404_STABLE, mariadb, false)	3m 11s
test (8.2, MOODLE_404_STABLE, pgsql, false)	3m 19s
test (8.2, MOODLE_404_STABLE, mariadb, false)	3m 12s
test (8.3, MOODLE_404_STABLE, pgsql, false)	3m 20s
test (8.3, MOODLE_404_STABLE, mariadb, false)	3m 18s
test (8.1, MOODLE_405_STABLE, pgsql, false)	3m 25s
test (8.1, MOODLE_405_STABLE, mariadb, false)	3m 13s
test (8.2, MOODLE_405_STABLE, pgsql, false)	3m 18s
test (8.2, MOODLE_405_STABLE, mariadb, false)	3m 7s
test (8.3, MOODLE_405_STABLE, pgsql, false)	3m 12s
test (8.3, MOODLE_405_STABLE, mariadb, false)	3m 30s
	46m 19s

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