Update item count and totals on Order#merge!

Fixes #4610
Capybara helpers should be available in extensions by default.
Remove unused Variant#gross_profit method
Memoize Order.tax_zone.

In our app, this speeds up GET /checkout/update/address
from 1 minute, 15 seconds to 2 seconds.

Fixes #4676

Conflicts:
	core/app/models/spree/order.rb