We should better display creation/updated at/plant before dates on seed trades

[CloCkWeRX]

http://growstuff.org/ and http://growstuff.org/seeds/oakandsage-yardlong-bean

On the home page "Seeds available for trade" section, it's difficult to tell if those seed trades are valid or recent (though the 'plant before' dates are blank).

We should consider rendering when the seed was made available for trade, or if present, the plant before date.