More initialize tweaks (#2971)

changes:

Organs now use Initialize().
The observe message now shows the configured respawn delay instead of always 30 minutes.
Seed datums no longer have a timer in New().
External organs no longer have a timer in New()/Initialize()
Converted some spawns in organs to timers.
Floor init no longer breaks to plating first for mapped in tiles.
Added the INITIALIZE_IMMEDIATE macro to make Initialize() not wait for SSatoms init.
Mannequins now use INITIALIZE_IMMEDIATE instead of a custom-defined New().
Mannequins (from get_mannequin) now delete themselves if unused for 5 minutes or longer.

code/controllers/subsystems/mob.dm

@@ -55,3 +55,10 @@
 	if (!.)
 		. = new /mob/living/carbon/human/dummy/mannequin
 		mannequins[ckey] = .
+
+	addtimer(CALLBACK(src, .proc/del_mannequin, ckey), 5 MINUTES, TIMER_UNIQUE | TIMER_OVERRIDE)
+
+/datum/controller/subsystem/mobs/proc/del_mannequin(ckey)
+	var/mannequin = mannequins[ckey]
+	qdel(mannequin)
+	mannequins -= ckey