-spec on_run_level(fun((enter | leave,
run_level()) -> _),
classy_hook:conf()) -> classy_hook:hook().
Register a hook that is executed on change of the run level of the local site.
When run level increases, hooks with higher priority run earlier; when it decreases, hooks run in the reverse order.
WARNING: if the callback interacts with the OTP application controller
(e.g. it starts or stops an OTP application),
then stopping classy application using application:stop(classy) will lead to a deadlock.
Use classy:stop_system() function to safely lower the run level and shut down classy.