Allow different remote translation keys per tenant

Note we don't need to update the tests; the tests themselves help us
confirm that `Rails.application.secrets` and `Tenant.current_secrets`
return the same object on single-tenant applications.

app/controllers/concerns/remotely_translatable.rb

@@ -26,6 +26,6 @@ module RemotelyTranslatable
     end
 
     def api_key_has_been_set_in_secrets?
-      Rails.application.secrets.microsoft_api_key.present?
+      Tenant.current_secrets.microsoft_api_key.present?
     end
 end