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 searching and paging for admin promotions area

Fixes #4478