Improves promotion#products collection method

promotion#products should return products from all rules that respond to
a products method and not just from the built in PromotionRule::Product
class.

This should make it easier for developers to use their custom rules to
interact with Spree::Promotion

Fixes #2363
move gateway form required chars "*"

close to their labels to uniform with the rest of forms

Fixes #2362
Fixed issue with duplicate thumbnails on ProductController#show

Fixes #2361