Admin secrets refactor.

Turns the entries in the secret admin secrets panel into datums, which will make it much easier to add, edit, and remove new entries.
This initial commit only contains an initial proof of concept entry implementation and the supporting code. Next commit will contain all relevant entries.

code/modules/admin/secrets/gravity.dm

@@ -0,0 +1,29 @@
+/**********
+* Gravity *
+**********/
+/datum/admin_secret_item/random_event/gravity
+	name = "Toggle station artificial gravity"
+
+/datum/admin_secret_item/random_event/gravity/execute(var/mob/user)
+	. = ..()
+	if(!.)
+		return
+
+	if(!(ticker && ticker.mode))
+		user << "Please wait until the game starts!  Not sure how it will work otherwise."
+		return
+
+	gravity_is_on = !gravity_is_on
+	for(var/area/A in world)
+		A.gravitychange(gravity_is_on,A)
+
+	feedback_inc("admin_secrets_fun_used",1)
+	feedback_add_details("admin_secrets_fun_used","Grav")
+	if(gravity_is_on)
+		log_admin("[key_name(usr)] toggled gravity on.", 1)
+		message_admins("\blue [key_name_admin(usr)] toggled gravity on.", 1)
+		command_announcement.Announce("Gravity generators are again functioning within normal parameters. Sorry for any inconvenience.")
+	else
+		log_admin("[key_name(usr)] toggled gravity off.", 1)
+		message_admins("\blue [key_name_admin(usr)] toggled gravity off.", 1)
+		command_announcement.Announce("Feedback surge detected in mass-distributions systems. Artificial gravity has been disabled whilst the system reinitializes. Further failures may result in a gravitational collapse and formation of blackholes. Have a nice day.")