Fix halo sometimes remaining when a unit is downgraded or killed

This is the rest of pull request #2028. Credit for these changes belongs to
@newfrenchy83.

I initially rejected these changes, but
* for advancement, @Arcanister pointed out that {ADVANCE_UNIT} is sometimes
  used to *downgrade* an unit. Thus, it's plausible that "advancing" can
  remove a halo.
* for killing, @newfrenchy83 said that it's *possible* (although unlikely)
  that the halo can remain. Even though I haven't reproduced the problem
  myself, it's good to ensure that the whiteboard releases its reference to
  the unit anyway.

Pull request #2028 has now been fully merged.

src/actions/advancement.cpp

@@ -353,6 +353,7 @@ void advance_unit(map_location loc, const advancement_option &advance_to, bool f
 		LOG_CF << "Added '" << new_unit->type_id() << "' to the encountered units.\n";
 	}
 	resources::gameboard->units().erase(loc);
+	resources::whiteboard->on_kill_unit();
 	u = resources::gameboard->units().insert(new_unit).first;
 
 	// Update fog/shroud.

src/actions/attack.cpp

@@ -1289,6 +1289,7 @@ void attack::unit_killed(unit_info& attacker,
 	}
 
 	units_.erase(defender.loc_);
+	resources::whiteboard->on_kill_unit();
 
 	// Plague units make new units on the target hex.
 	if(attacker.valid() && attacker_stats->plagues && !drain_killed) {