Validate map templates uploaded by admins (#39674)

* Move the preloader datum to its own file

* Prettify some of the map loader docs

* Use src rather than usr in map template verbs

* Cache parsed templates between upload and first use

* Validate map templates uploaded by admins before use

* Add href token to validation report links

code/modules/mapping/preloader.dm

@@ -0,0 +1,31 @@
+// global datum that will preload variables on atoms instanciation
+GLOBAL_VAR_INIT(use_preloader, FALSE)
+GLOBAL_DATUM_INIT(_preloader, /datum/map_preloader, new)
+
+/// Preloader datum
+/datum/map_preloader
+	parent_type = /datum
+	var/list/attributes
+	var/target_path
+
+/datum/map_preloader/proc/setup(list/the_attributes, path)
+	if(the_attributes.len)
+		GLOB.use_preloader = TRUE
+		attributes = the_attributes
+		target_path = path
+
+/datum/map_preloader/proc/load(atom/what)
+	GLOB.use_preloader = FALSE
+	for(var/attribute in attributes)
+		var/value = attributes[attribute]
+		if(islist(value))
+			value = deepCopyList(value)
+		what.vars[attribute] = value
+
+/area/template_noop
+	name = "Area Passthrough"
+
+/turf/template_noop
+	name = "Turf Passthrough"
+	icon_state = "noop"
+	bullet_bounce_sound = null