Use flash[:success], not flash.success

Fixes spree/spree#3166

Conflicts:
	app/controllers/spree/admin/users_controller.rb
Only check if user is an admin *before* they are created, not all the time on save

This was otherwise causing issues in our tests
stub spree_current_user, not just current_user, within admin/shipments_controller spec
Correct link click for user in user_spec
Set default_url_options to appease password_reset_spec test