Use I18n for variants on product creation prototype listing

Fixes #3844
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
Use sRGB colorspace

Fixes #3843
Fixes #3104

Conflicts:
	core/app/models/spree/image.rb