Only set display_name in OrderPopulator#check_stock_level when it's needed

This should speed up in_stock query checking, as it's no longer querying option_values + option_types tables as well.
Move Error Text calculation inside error block
Fixed bad style escaping within admin/products/index.html.erb

Fixes #3439
Do not check stock quantity twice when saving line item

Every time an user pressed the add to cart button spree was checking
stock in OrderPopulator and before saving the line item, through
Stock::AvailabilityValidator. I believe we only need to check stock on
the LineItem model

Fixes #3434

Conflicts:
	core/app/models/spree/order_populator.rb
Add validation of shipping_category_id to Product. Fixes #3188
README fix - use example that's still relevant
Remove select2-rails from Rails frontend gem

Bump select2-rails to ~> 3.4.1 in backend gem

Update helper `select2_search` implementation due to latest changes on
select2 generated markup, had to find a new way to grab select2 element
id

Fixes #3457
Remove rails from frontend and backend gemspecs.
Remove deface from frontend and backend gemspecs.