Nightshifts & Randomized Station Time

This commit adds the Nightshift lighting mode from /tg/. From 19:00 to
07:00 in station time, the station will go into a reduced-lighting mode,
with dimmed lights across the station.

In conjunction with this, there is a new configuration option to start
the shift time at a random time other than 12:00, so that players are
more likely to experience a night shift (as opposed to having to have a
7 hour round).

code/modules/store/store.dm

@@ -41,7 +41,7 @@ var/global/datum/store/centcomm_store=new
 	T.purpose = "Purchase of [item.name]"
 	T.amount = -amount
 	T.date = current_date_string
-	T.time = worldtime2text()
+	T.time = station_time_timestamp()
 	T.source_terminal = "\[CLASSIFIED\] Terminal #[rand(111,333)]"
 	mind.initial_account.transaction_log.Add(T)
 	return 1