Persist address ids on UserAddress module

Otherwise addresses would not be associated with the user at all
Replaced Taxon :position refs with :child_index=

Fixes #3390
Fixes #3956

Taxon position unnecessarily fights against the awesome_nested_set
API. By using a_n_s's :move_to_child_with_index method we can more
fully leverage acts_as_nested_set.

Conflicts:
	core/app/models/spree/taxon.rb
	core/spree_core.gemspec

Conflicts:
	backend/app/controllers/spree/admin/taxons_controller.rb
Bump aws-sdk to 1.27.0 to remove restriction for Nokogiri < 1.6
Remove position and child_index from taxon_attributes, since they're no longer used