[api] Fix 1.8 syntax error in Api::V1::BaseController
Fix 1.8 incompatibility with Time.new in base_helper_spec

Related to #2405
make new product button always visible on edit product page.

I found that it will be very useful when entering products through admin side. this will allow use to quickly navigate to product page after adding new one without going back to product list

Fixes #2407
Namespace OrderPopulator reference in OrdersController so people don't run into potential issue where it would attempt to lookup a top-level OrderPopulator class
Fixed hardcoded paths to fontawesome

Fixes #2424
Remove unused translation for no_items_in_cart + order_confirmation_note
Switched decorator loading to the decorators gem.

Part of #2415
Moved decorators under the app/decorators convention.

Part of #2415
Moved Spree::StoreController class_eval from engine to store_controller_decorator.

Part of #2415
Revert changes from #1981

This is due to sbounmy suggesting it could be useful:
https://github.com/spree/spree/issues/1981#issuecomment-11682782
Use available? and acknowledge allow_backorders setting in OrderPopulator

Fixes #2430