Fix MOTD

It now won't require there to be at least one entry in the motd table.
I also tidied up the motd class

dist/pvv_mysql.sql

@@ -31,6 +31,7 @@ CREATE TABLE users (
 );
 
 CREATE TABLE motd (
+`id` INTEGER PRIMARY KEY AUTO_INCREMENT,
 `title` TEXT,
 `content` TEXT
 );