Fixed Spree::Api::ProductsController#index to accept multiple ids

Fixes #3886
Relax money dependency
Bugfix: Taxons could not be ordered

If you reorder the taxons in the admin interface, it does not
have any effect, because the ordering of the taxons is not
used. Setting a default scope with this ordering fixes that.

Fixes #3891
Fixes #3833
Fixes #3390
Spree::Payment.class.undescore will always convert spree/payment

Fixes #3889

Conflicts:
	backend/app/views/spree/admin/payments/show.html.erb
Fix fieldset not resizing on some @media sizes.

Fixes #3871
Fixes #3890
[api] Don't depend on ordering in taxons_controller_spec
[api] Add missing per_page node to products/index