Make sure checkout payment only flow works

Fixes #3142
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
Correct template used for backend assets in install generator

Thanks to @alepore's comment here: https://github.com/spree/spree/commit/cfbd0f3#commitcomment-4865755