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
Move font-awesome into app/assets for scss import. Fixes #3465
Fix `load_sample`

Broke after 7a5e43662c57807dafaebf8160069e4de8f1c855 which requires
product to have a shipping category

Add build for spree/sample load data task
Add Sample to build.sh
Additional can?'s in product admin

* Hide new/edit/clone/delete product links if they are unauthorized
* Hide any tabs from the product sidebar which are unauthorized

Allows fine grained control over which actions on a product are
permitted.
This already worked properly through use of authorize! in controllers.
However this left dead links to unauthorized pages.

Additional can?'s in Order admin

* Hide new order button if unauthorized
* Hide events, resend button if unauthorized

custom_authorization! test helper

Creates a new custom_authorization! test helper to aid in defining
custom cancan rules within test cases. Replaces existing uses of
stub_bar_authorization! with custom_authorization! and explicit rules.

Feature specs of hidden unauthorized links

Fixes #3469
Add require spree_sample to spec helper to prevent failure when running build.sh.
Store tax category on line item

Fixes #3482
Fixes #3481
Remove padding from date-range-filter arrow

Relates to #3465
Fix issue where admin nav would display in a Times-like font

Related to #3465
Remove deprecated -moz-border-radius clause (deprecated in Firefox 13)
Fix styling of select2's droparrow
Add changelog entry for #3481