Convert db seed + sample files to Ruby

This removes some confusion about what countries are in what zone members, and will prevent us from running into issues like #2547
[frontend] Remove duplicate  call for spree/orders in CheckoutController
changed products_path to spree.products_path

Fixes #2559
changed root_path to spree.root_path to allow template to be recalled

Fixes #2559
Testing support files have been refactored.

Fixes #2552
Removed unnecessary ternaries

Fixes #2554
added specs to show how to add address to Order through API and improved addresses retorn with error

Fixes #2546
Fixes #2549
Refactors load_order in checkout controller

Creates one method for each logic requiring a redirect and make it a
before_filter. Hopefully making it easier to view that logic and change
it

Also removes invalid comment about #update being updated in promo. Since
there is no promo gem any longer
Removes #state_callback from checkout controller

Avoids confusion scenarios such as the one described in #2488. After
state callback methods does not make much sense at all. Besides it
doesn't behave as expected. Fixes #2488

Fixes #2557
to address in mail_interceptor is always an array. Remove brackets.
Convert db seed + sample files to Ruby

This removes some confusion about what countries are in what zone members, and will prevent us from running into issues like #2547
Remove Spree::Core::CustomFixtures + related code

This deprecates being able to load data using yml with load_file and load_dir Rake tasks