Doubles time between mail shipments and blocks mail on red / delta alert (the amount of mail is not changing don't worry) (#91869)

## About The Pull Request

1. Mail is now shipped every 2 SSeconomy ticks, rather than every
SSeconomy ticks. The amount of mail per round will not change.

2. Mail is not shipped during Red or Delta alert. Mail will still
continue to pile up for if the station returns to blue alert.

## Why It's Good For The Game

1. The time between cargo shipments is like five minutes. This means you
get mail practically every cargo shipment, if only a few pieces. I think
it's a bit spammy, so doubling time between shipments means you actually
get substantial shipments of mail and you don't have to pay attention to
mail constantly.

2. In the event of an emergency and the cargo shuttle has to fly back
and forth for emergency supplies (like guns for a blob), being blocked
because of mail is annoying. It's also reasonable from an in character
perspective that they would hold your mail until the emergency is
resolved.

## Changelog

🆑 Melbert
qol: Mail is now shipped every 10 minutes rather than every 5 minutes.
The amount of mail remains unchanged (so you get 1 crate with 6 mail
instead of 2 crates with 3 mail each).
qol: Mail is no longer shipped while the station is on red alert - mail
held will be sent if the station returns to blue alert.
/🆑

code/modules/shuttle/mobile_port/variants/supply.dm

@@ -306,7 +306,7 @@ GLOBAL_LIST_INIT(blacklisted_cargo_types, typecacheof(list(
 */
 /obj/docking_port/mobile/supply/proc/create_mail()
 	//Early return if there's no mail waiting to prevent taking up a slot. We also don't send mails on sundays or holidays.
-	if(!SSeconomy.mail_waiting || SSeconomy.mail_blocked)
+	if(!SSeconomy.mail_waiting || SSeconomy.mail_blocked || SSsecurity_level.current_security_level.disables_mail)
 		return
 
 	//spawn crate