Add countries to state selection for zones

When editing a state based zone it is impossible to determine the
country each state is in. For example, Maryland, Liberia and Maryland,
USA are identical options. This is very confusing for the user and can
lead to mishaps.

Author: jacobherrington

backend/app/views/spree/admin/zones/_state_members.html.erb

@@ -4,7 +4,7 @@
 
     <%= zone_form.field_container :state_ids do %>
       <%= zone_form.label :state_ids, plural_resource_name(Spree::State) %><br>
-      <%= zone_form.collection_select :state_ids, @states, :id, :name, {}, { multiple: true, class: "select2 fullwidth" } %>
+      <%= zone_form.collection_select :state_ids, @states, :id, :state_with_country, {}, { multiple: true, class: "select2 fullwidth" } %>
     <% end %>
   </fieldset>
 </div>

core/app/models/spree/state.rb

@@ -36,5 +36,9 @@ def <=>(other)
     def to_s
       name
     end
+
+    def state_with_country
+      "#{name} (#{country})"
+    end
   end
 end