Change “Translate” button to only show up when a translation backend is configured (#19434)

* Change “Translate” button to only show up when a translation backend is configured

Fixes #19346

* Add `translation` attribute to /api/v2/instance to expose whether the translation feature is enabled

Fixes #19328

app/serializers/rest/instance_serializer.rb

@@ -72,6 +72,10 @@ class REST::InstanceSerializer < ActiveModel::Serializer
         min_expiration: PollValidator::MIN_EXPIRATION,
         max_expiration: PollValidator::MAX_EXPIRATION,
       },
+
+      translation: {
+        enabled: TranslationService.configured?,
+      },
     }
   end