Fixes sending stuff to "Old" Chat (tgstation#79819)

## About The Pull Request

This functionality was removed in tgstation#79479
(e1c6cfd), and we should still be
supporting the old chat anyways because it contains a plethora of useful
BYOND information that we still can really leverage (such as the
built-in profiler and stuff like that) and it's going to be painful to
do that if you have to keep spamming `fix-chat` to see OOC/ASAY while
alternating every damn time.
## Why It's Good For The Game

It's ugly but we still need it. There's a reason why we still have it.
## Changelog
:cl:
fix: "Old Chat" (AKA: The old-styled non-TGUI raw-HTMLesque chat that
you might see when it prods you with the "Failed to load fancy chat!"
issue) should now get all text messages as expected.
/:cl:

code/controllers/subsystem/chat.dm

@@ -43,6 +43,7 @@ SUBSYSTEM_DEF(chat)
 
 /datum/controller/subsystem/chat/proc/send_payload_to_client(client/target, datum/chat_payload/payload)
 	target.tgui_panel.window.send_message("chat/message", payload.into_message())
+	SEND_TEXT(target, payload.get_content_as_html())
 
 /datum/controller/subsystem/chat/fire()
 	for(var/ckey in client_to_payloads)

code/datums/chat_payload.dm

@@ -10,3 +10,7 @@
 /// Converts the chat payload into a JSON string
 /datum/chat_payload/proc/into_message()
 	return "{\"sequence\":[sequence],\"content\":[json_encode(content)]}"
+
+/// Returns an HTML-encoded message from our contents.
+/datum/chat_payload/proc/get_content_as_html()
+	return message_to_html(content)