Adds 'current order' API endpoint

This enables an API client to load a user's latest order. At Bonobos, we
use this to load the user's last active cart.

The find_current_order method is broken out to a separate method to
enable gems like spree-multi-store to customize its behavior.

Closes #5069
Merge pull request #9 from gmacdougall/2-3-dev

Adds 'current order' API endpoint