Fix emoji exploit (#640)

## Description:
This should fix the exploit that allows players to send custom text as
an "emoji". It does this by introducing a emoji ID (index into the emoji
table) instead of sending the raw emoji as a string.

## Please complete the following:

- [ ] I have added screenshots for all UI updates
- [X] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced
- [X] I understand that submitting code with bugs that could have been
caught through manual testing blocks releases and new features for all
contributors

## Please put your Discord username so you can be contacted if a bug or
regression is found:
PilkeySEK

src/core/Schemas.ts

@@ -9,6 +9,7 @@ import {
   PlayerType,
   UnitType,
 } from "./game/Game";
+import { flattenedEmojiTable } from "./Util";
 
 export type GameID = string;
 export type ClientID = string;
@@ -133,14 +134,10 @@ const SafeString = z
   )
   .max(1000);
 
-const EmojiSchema = z.string().refine(
-  (val) => {
-    return /\p{Emoji}/u.test(val);
-  },
-  {
-    message: "Must contain at least one emoji character",
-  },
-);
+const EmojiSchema = z
+  .number()
+  .nonnegative()
+  .max(flattenedEmojiTable.length - 1);
 const ID = z
   .string()
   .regex(/^[a-zA-Z0-9]+$/)