Disallow users from inviting others to private modmail threads (kangalio#25)

Should close kangalio#15.

Author: wackbyte

src/commands/modmail.rs

@@ -179,6 +179,11 @@ async fn create_modmail_thread(
 		)
 		.await?;
 
+	// disallow users from inviting others to modmail threads
+	modmail_thread
+		.edit_thread(ctx, |edit_thread| edit_thread.invitable(false))
+		.await?;
+
 	let thread_message_content = format!(
 		"Hey {}, {} needs help with the following:\n> {}",
 		ctx.data().mod_role_id.mention(),