Do-over "Add Stock Movement" form

If there are a large number of stock items for this stock location, the page can take some seconds to load, as mentioned in this thread:

https://groups.google.com/forum/#!topic/spree-user/nu0_NlNGPV8

To make that stop being a problem, I've switched that UI to using select2 to load stock items 50 at a time. I've also imporved the UI to show more information about the product, such as its SKU and its options text.
[backend] Include options text when displaying variants on the stock movements page
[api] Add eager loading for option values + option types to stock items

Conflicts:
	api/app/controllers/spree/api/stock_items_controller.rb
[api] include option_type presentation within option_value_attributes
[backend] More clearly differentiate select2 result elements
[backend] Split variants/_autocomplete into two templates which can be used independently