Better press timing

The `Message.sentAt` is not in sync with the time on the server. (Well on my machine anyway)

This makes the timing of the press times more reliable by sticking to the boxes clock, which should all be in sync.

This adds `creationTime` to the `PressJob` case class and is carried through the queue.
`SentAt` is still being requested from `SQS`; actually I think I will remove it.

@robertberry 
https://github.com/guardian/frontend/blame/master/facia-press/app/frontpress/ToolPressQueueWorker.scala#L53