Revert "Remove call to authorize! which is actually not doing anything"

Need to call authorize with :admin to prevent index on Product which is
allowed in the ability here:

https://github.com/spree/spree/blob/master/core/app/models/spree/ability.rb#L57

NOTE: The ability intends the permission for the front_end but it
applies to the back end also.

This reverts commit a1d2e6f018858ad8d7a7b5cdd7f77307962feff3.

Conflicts:
	core/app/controllers/spree/admin/base_controller.rb
	core/lib/spree/core/testing_support/authorization_helpers.rb