adding spec and fixing user_logged_in promo rule

Fixes #2884
Add short explanation to tax rate amounts to ease confusion
Greatly reduce number of drawn routes on Spree::Core::Engine

See explanation in https://github.com/radar/spree/commit/c7c260278b3de9660289c756104deef5298c5082
Guard against admin js files being loaded outside of the actual admin

This can happen if application.js has a //= require_tree . directive
Bump Modernizer to 2.6.2
Enforce usage of in screen.css.scss and add a new variable to the variables.css.scss file to allow for an easier customization

Fixes #2933