Fix forums to avoid double posts

[birdofpreyru]

topcoderinc/cab#49

Sometimes it happens that if you click accidentally twice on the post button you create unintentional double posts, it would be nice to check for double posts and avoid them. For example, don't let a user post more than once in a 2 seconds time range.

Time delay is not a proper solution though. Forum API, when a user POSTs a new message, should check that the message payload (or the destination thread) is different from that of the last message by the same user.