provide consistent sort order for promotions

make pagination sane for promotions.
especially true for postgres which seems to do more to keep its promise that "If sorting is not chosen, the rows will be returned in an unspecified order".
since there are no indexes on this table we just use the primary key with most recent first.

Fixes #4596

Conflicts:
	backend/spec/controllers/spree/admin/promotions_controller_spec.rb