add searching and paging for admin promotions area

This is a cherry-pick of a commit that I previously got added to spree/2-2-stable: 9606cff2a42684bfca64e80f2ecc800adec2dc0c

I did a quick test locally and it seems to work.  The specs also still pass for me.

Conflicts:
	backend/app/controllers/spree/admin/promotions_controller.rb