API: slots_add_slot_filter

for supervisors

additive, i.e. needs to be called once per filter to add.
filters are ORed, so technically the opposite of a filter… more like holes in an existing filter that doesn't let anything through. "additives"? …whatever
if filter refers to a course, check if user is teacher in said course note for later: if (!has_capability(user_helper::CAPABILITY_TEACHER, $context, $user))

necodeIT / lb_planner_plugin