remove horizontalNav js from admin this extension was causing warnings on the js consoles since it was using outdated jquery functions, and hasn't been updated this js plugin was here to allow ie7 < users to see the backend navigation using full-width

Fixes #3720
fix the ActiveRecord::ReadOnlyRecord exception when the @product is null when update the variant

Fixes #3718

Conflicts:
	api/spec/controllers/spree/api/variants_controller_spec.rb
Fix ReadOnlyError when destroying StockItem

StockMovement is a readonly object so we can't delete them. And by that
logic I believe we shouldn't clear stock items from the database either
otherwise we will have stock movements associated with non existing
stock items

Fixes #3727
Fixes #3723