Only count eligible promotions towards Promotion#credits

Fixes #4120

Conflicts:
	core/app/models/spree/promotion.rb
[api] Split taxon_ids as they're passed to Products#create and Products#update
make sure we do the line_item adjustment check before rendering the adjustment

Fixes #4122
Fixes #4130
fix backend layout assets tags

Fixes #4127
Render cart link directly to the page

Deciding to go with fragment caching for the time being
Make view cache key for products consistent

Now show views and lists of products both start with views/spree/products