Security: Added double-post/spambot checker

I've added a feature to the posting form which checks if the request is actually a new post from the shoutbox or just another post using an old session.

This stops two annoyances:

• People clicking the refresh-button in the browser and therefore post the same message again


• Spambots using an old session to make multiple posts (now you actually have to view the shoutbox before posting)

I specially hope this will keep most spambots away.