Dont advance order state when adding an item to cart

As of d02f67dbbf7e8ae1f61b65066c2a3a8234796a11

Admin would click to add a item to the order which would create the line
item and shipment but because we advance the order to address state the
shipments would be destroyed Order#ensure_available_shipping_rates

Cant see why we needed advance order in the first place.
Verify inventory when target_shipment is provided
Fix 500 when updating order email to something inconsistent with the associated user's email

Fixes #4614
Rename poorly named spec

Fixes #4616
There's no orders#index in spree_frontend
Lazyload variant associations in api/variants
Remove useless return in the show action from the products controller

Fixes #4635