[Zones] Convert IDLE_WHEN_EMPTY to a Zone Column (#3891)

* [Rules] Convert IDLE_WHEN_EMPTY to a rule

# Notes
- Converts `IDLE_WHEN_EMPTY` to `Zone:ZonesIdleWhenEmpty` so that we can change this on the fly or on a zone-by-zone basis instead of having to recompile to do this.
- Especially helpful for those using release binaries that do not compile their own source.

* Convert to zone column.

* Update ruletypes.h

* Update ruletypes.h

* Update entity.cpp

* Update entity.cpp

* Rename.

* Update database_update_manifest.cpp

* Update base_zone_repository.h

* Update zone.cpp

* seconds_before_idle

* Update database_update_manifest.cpp

* Getter/Setters/Private

* Update base_zone_repository.h

* IsIdle()/SetIsIdle()

* Update entity.cpp

zone/questmgr.h

@@ -106,7 +106,7 @@ public:
 	void depopall(int npc_type = 0);
 	void depopzone(bool StartSpawnTimer = true);
 	void repopzone();
-	void processmobswhilezoneempty(bool on);
+	void processmobswhilezoneempty(bool idle_when_empty);
 	void settarget(const char *type, int target_id);
 	void follow(int entity_id, int distance);
 	void sfollow();