Two new official posters (tgstation#72783)

## About The Pull Request
Adds two new official posters to the game.


![image](https://user-images.githubusercontent.com/10399117/213078165-f6e443a1-19e7-41bd-a915-baf7a8a21caf.png)

One poster teaches the crew about common 'kill words' they can use to
defeat enemy agents, the other teaches the crew to keep corporate
secrets where they belong: in the company. You wouldn't download a gun,
right?

## Why It's Good For The Game

These feel like thematically appropriate additions you might see on a
corporate-created poster, and are good references to space stations
past, and our favorite copyright memes.

## Changelog

:cl:
imageadd: Two new corporate themed posters.
/:cl:

code/game/objects/effects/poster.dm

@@ -1061,4 +1061,14 @@ MAPPING_DIRECTIONAL_HELPERS(/obj/structure/sign/poster/official/random, 32)
 	desc = "A poster that informs of active holidays. None are today, so you should get back to work."
 	icon_state = "holiday_none"
 
+/obj/structure/sign/poster/official/boombox
+	name = "Boombox"
+	desc = "An outdated poster containing a list of supposed 'kill words' and code phrases. The poster alleges rival corporations use these to remotely deactivate their agents."
+	icon_state = "boombox"
+
+/obj/structure/sign/poster/official/download
+	name = "You Wouldn't Download A Gun"
+	desc = "A poster reminding the crew that corporate secrets should stay in the workplace."
+	icon_state = "download_gun"
+
 #undef PLACE_SPEED