Incorrect use of _state instead of _states for I18n in state changes

[leemour]

There is a mixup or singular/plural _state/_states forms in views.

I have config.action_view.raise_on_missing_translations = true
When accessing order state_changes ( e.g. /admin/orders/R074278336/state_changes) I get an error:

I18n::MissingTranslationData in Spree::Admin::StateChanges#index
Showing /home/leemour/.rbenv/versions/2.4.2/lib/ruby/gems/2.4.0/gems/spree_backend-3.4.1/app/views/spree/admin/state_changes/index.html.erb where line 25 raised:
translation missing: ru.spree.shipment_state.backorder

In locale files, shipment_state is a string, not a hash containing backorder and translates to the equivalent of 'Shipment State'.
There is shipment_states (plural form) entry in locale files that contains backorder and this is the right I18n scope that should be used. The same is with payment_states.
Singular forms (_state) are used in views for translation and are supposed to have string value as in locale files. While plural forms (_states) are used as scopes to translated nested entries (states). There is just another mixup with order_state though, it should be changed to order_states in locale files to be consistent.

I'll include a PR with fixes that make this I18n scopes consistent. I hope you will accept it.
#8512

[githubble01]

@damianlegawiec and @leemour: After having worked on #854 in spree_i18n, I really think that resolving this issue would be of great value to all the translation contributers out there.

How would we go about this?
BR