Make sure checkout payment only flow works

Fixes #3142

Conflicts:
	frontend/app/views/spree/checkout/payment/_gateway.html.erb
Remove Spree::LegacyUser.anonymous! and anonymous?

These methods seem to have no more use within Spree.
Add EmailValidator#valid?

This method can be used outside the scope of an AR instance to validate if an email is indeed valid or not

Used within spree_auth_devise's CheckoutController decorator as of 29d167655314961e1043ae45694528b8f364c3fc
make sure we load the promotion rules after initialize so the user rules are correctly saved. also remove the after_save on promotion, this is no longer needed.
[api] Passing without_children=1 to taxons endpoint will now return a 'shallow' result set without children

Fixes #4112
Only show orders/mine for persisted users
[backend] remove reference to removed anonymous? method in customer_details/form