Return html_safe strings from variant_price_diff

Fixes #2737

Conflicts:
	core/app/helpers/spree/products_helper.rb
Explicitly setting taxon edit path in the form

If not made this way then taxon.permalink is used in the url model thus raising unknow path error.
add failling test for https://github.com/spree/spree/issues/2823

Fixes #2869