fix for report locale names when extending via class_eval

* source issue: https://github.com/spree/spree/issues/4086
* try to maintain the reports not in a constant
* make it easier to add new reports over current merging arrays way

Example decorator now becomes:

Spree::Admin::ReportsController.class_eval do
  add_available_report! :some_report

  def some_report

  end
end