Adds config for roundstart blue alert (#92015)

## About The Pull Request

Adds a config option `roundstart_blue_alert` which determines if the
station is put on blue alert on roundstart.

**Greenshifts** are unaffected, they will still have a unique
announcement indicating it's a greenshift

**Roundstart reports** are unaffected, they will be sent regardless. 

## Why It's Good For The Game

Some servers put more player agency on command to control the report
level, some servers re-theme the levels so blue is more of an involved
thing, some servers put more weight on the current level, etc. Giving
the option of staying on green until someone decides to up it is neat I
guess.

Before doing this I tried to find when this was even added - to see what
the justification was and make sure I wasn't violating it - and wasn't
successful. R4407 had "`Security Level Elevated`" in their reports but
didn't have security levels implemented, so it was purely fluff.

## Changelog

🆑 Melbert
config: Adds "roundstart_blue_alert", allowing you to disable roundstart
blue alert. Defaults to "on".
/🆑

code/controllers/configuration/entries/game_options.dm

@@ -178,6 +178,9 @@
 /datum/config_entry/string/alert_delta
 	default = "Destruction of the station is imminent. All crew are instructed to obey all instructions given by heads of staff. Any violations of these orders can be punished by death. This is not a drill."
 
+/datum/config_entry/flag/roundstart_blue_alert
+	default = TRUE
+
 /datum/config_entry/flag/revival_pod_plants
 
 /datum/config_entry/number/revival_brain_life