Stop validating booking not in past @ server side

* This is validated already @ client side by js, there we
use browser timezone.
* Don't validate server side anymore because there we don't know
the correct timezone for the marketplace. Before this change we've
used single server timezone to determine what "today" means, which is
obviously a bad idea for most marketplaces.
Merge pull request #822 from sharetribe/no-todayvalidation-booking

Stop validating booking not in past @ server side