Configurable chat history length

The previous default of 5 messages is utterly lame and didn't provide enough context to anything, and was also a downgrade from the old server as that had 10 messages by default (of which we're now going back to with this change).

src/IConfig.ts

@@ -21,6 +21,7 @@ export default interface IConfig {
         moderatorEnabled : boolean;
         usernameblacklist : string[];
         maxChatLength : number;
+        maxChatHistoryLength : number;
         automute : {
             enabled: boolean;
             seconds: number;