public.json: Add /order-lines and /order-line/{id}

I've simplified our existing OrderLine model for the JSON API.
Changes:

* Dropped 'line_sequence_number'.  I suppose it's possible that folks
  would care about this, but I'm not sure its important enough to be
  worth a separate field in the API.
* Replaced 'average_landed_weight_at_checkout_time' and 'line_weight'
  with 'weight', which records the total picked weight for the line.
  The old fields just recorded the generic weights, not the actual
  picked weights.
* Dropped 'line_volume'.  Volumes are interesting on a per-trip level
  for allocating trucks and on a per-order/piece level for packing
  boxes.  Neither one of those is a per-order-line level.
* Dropped assorted cached fields.  We can add them back if follow-up
  requests turn out to be too slow.

I was just discussing the price lock-in with Benjamin on IRC (20:00 to
20:55 PST).  Summary of our discussion:

* Once we get 24-hour-before-cutoff stock allocation, we'll float the
  price until then (which will allow customers to respond to
  out-of-stock and unacceptable-price-change issues).
* If you resume an order, you only lose your earlier locked-in prices
  if you increase the ordered quantity.  We should likely warn users
  in this situation and suggest they start a new order line at the new
  price.
* Prices are locked at line-placement time, but overstock counts are
  currently decremented at pick time.  So if you are 10 counts over
  stock and folks place orders for 40, they all lock-in the overstock
  pricing.