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/initial_state_serializer.rb

@@ -31,6 +31,7 @@ class InitialStateSerializer < ActiveModel::Serializer
       timeline_preview: Setting.timeline_preview,
       activity_api_enabled: Setting.activity_api_enabled,
       single_user_mode: Rails.configuration.x.single_user_mode,
+      translation_enabled: TranslationService.configured?,
     }
 
     if object.current_account