Fix DEPRECATION WARNING: Calling #sum with a block is deprecated and will be removed in Rails 4.1.
Normalize Product#slug on update

friendly_id only generates the slug by default when it's nil. Spree
however allows users to set it directly in the backend and we need to
make sure a valid slug gets persisted otherwise users might run into
route errors trying to see the product

Fixes #4451
Set charset=utf-8 on api default Content-Type

Fixes #4423
Add Montenegro to Countries Seed

Montenegro is missing from the country seeds file. I pulled the information from the [ISO page for Montenegro](https://www.iso.org/obp/ui/#iso:code:3166:ME).

Fixes #4465
Add missing choose_location, existing_shipments and new_shipment_at_location translations
Add an initial store model

The attributes are all the ones that will be supported for the initial
preferences-onto-store refactoring

Related to #4415
Create a store on new/store migration

Grabbed the defaults from the existing preferences.

Related to #4415
Switch config editor to modify Spree::Store

Related to #4415
Switch examples off of site_name

With site_name now on store, we need to track something so it may as
well be track_inventory_level as an example

Related to #4415
Switch app config test to move off of site_name

Related to #4415
Switch Spree::Config to use new store attributes

Related to #4415
don't override existing configuration methods

Related to #4415
meta_keywords delegated to description
Updating for new preferences api

Broken by @jhawthorn's a36381edb9014a97c7d337cb1d85989672388428

Related to #4415