add support for custom keys in question variables and fix linkage to … #823

Job	Run time
test (8.2, master, pgsql, SBCL)	32m 55s
test (8.1, MOODLE_402_STABLE, pgsql, GCL)	31m 57s
test (8.0, MOODLE_402_STABLE, pgsql, SBCL)	25m 49s
test (7.4, MOODLE_401_STABLE, mariadb, GCL)	1h 47m 39s
test (7.4, MOODLE_400_STABLE, pgsql, SBCL)	23m 21s
test (7.3, MOODLE_311_STABLE, pgsql, SBCL)	23m 8s
test (7.2, MOODLE_39_STABLE, pgsql, SBCL)	28m 21s
	4h 33m 10s

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