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
Add another migration to link existing line items with their tax categories

Related to #3481