Remove unused spree_backend.js file leftover from asset migration.
Remove unused spree_backend.css file leftover from asset migration.
Add spree to extension gemfile to make frontend/backend available.
Fixes #4527
remove selenium gem and selenium conditionals, running on poltergeist perfectly now
Touch resources when updating positions
fix collection cache key with total models count
remove assign_stock_changes_to

Nobody likes this function. It showed up to a party once and everyone
else left. Its parents are ashamed of it. It listens to bad music.
Removing it is doing the whole class a favor.

Fixes #4532
[api] Support order routes on /api/checkouts as well as /api/orders

I got sick of not knowing if /api/checkouts was the right prefix to use, or if I should use /api/orders. If I get sick of it after one day of usage, then I'd suspect other people who are using the API would also be sick of it.

Let's fix this problem and there will be no more getting sick of it :)
[api] Allow for accessing states inside of countries in the API

Strangely, StatesController already supported this but there wasn't a route defined for that. I only found this out today when I tried making a request to /api/countries/:country_id/states (with my spree_api_examples project) and had that fail.
[order importer] Assign created shipping rate as the selected shipping rate
[order importer] Ensure that order totals are correctly updated
[api] Do not assume that params[:id] is for the order only
[backend] Make variant selection in _create_line_items wider
[backend] convert variant_autocomplete to coffeescript
[backend] Remove options_placeholder in favour of returning options in select2 result
[backend] only show options text in variant_autocomplete if there is text to display
Include display_tax_total in API responses
Wipe all shipments when ensure_available_shipping_rates validation fails
Touch an order when a line item is saved

Fixes #4419

Conflicts:
	core/app/models/spree/line_item.rb
	core/spec/models/spree/line_item_spec.rb
backend: Remove missing reports#show route

Fixes #4528