Give us a chance to skip adjustments on order import via api

Since adjustments is tightly coupled with line items creation we can
only do that by destroying tax adjustment after line items are created.
Add :send_core_emails config prefernce

Allow developers to use standard rails mail configuration (in config
files) by setting :override_actionmailer_config to false without sending
spree core emails (e.g. order confirmation). This is useful e.g. in the
case where devs have opted to use an external mail API such as Mandrill
for store-related emails but still want to use ActionMailer in other
parts of their app.

Fixes #3812
Ensure that preference keys are not prefixed with '/' if ENV['RAILS_CACHE_KEY'] is not set

Fixes #3831