[backend] Fix failing update_positions spec.

The update_positions spec failed due to it expected initial position to
be nil when created but its actually 1.
Allow stock to be transferred between stock locations
API - Fix missing translation for 'invalid taxonomy id'.

Invalid taxonomy id was not translated.
Ensure item total in checkout uses correct currency

Related to #2737
Only allow editing of order and shipments if ability permits it. Also scope shipment editing js to allow other shipment forms.

Fixes #2885
Account for current_order returning 'nil' in OrdersController#update

Fixes #2750
Replace stub! with stub.

See David Chelimsky's explanation: http://www.mail-archive.com/rspec-users@rubyforge.org/msg13270.html

Fixes #2898
Fix issue where unchecking a stock location could cause it to reuest /admin/stock_items/{id}

Fixes #2896