Add validation of shipping_category_id to Product. Fixes #3188

Conflicts:
	api/CHANGELOG.md
	api/app/helpers/spree/api/api_helpers.rb
	api/spec/controllers/spree/api/products_controller_spec.rb
	backend/spec/features/admin/orders/shipments_spec.rb
	core/CHANGELOG.md
	core/spec/models/spree/product_spec.rb
Fix spec broken by conflict.
Remove select2-rails from Rails frontend gem

Bump select2-rails to ~> 3.4.1 in backend gem

Update helper `select2_search` implementation due to latest changes on
select2 generated markup, had to find a new way to grab select2 element
id

Fixes #3457
index spree_orders#completed_at
Add logo back
Correct font-awesome weirdness in admin backend

Icons for edit + delete links were incorrectly positioned and admin navbar text was very thin
Add option_values to variant template [api]

Fixes #3460
[api] Passing no parameters to ShipmentsController#update should not make it 500
Added :user at :tab was missing in I18N

Fixes #3455