Expose moved_to_id in the issues API

In app/serializers/issue_entity.rb we do expose moved_to_id but we didn't do so in the API in lib/api/entities.rb. Something like this should do it:

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 4018ce900e1..38cb7d0f8f6 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -619,6 +619,11 @@ module API
     class Issue < IssueBasic
       include ::API::Helpers::RelatedResourcesHelpers
 
+      expose :moved_to_id, if: -> (issue, options) do
+        issue.moved? &&
+          Ability.allowed?(options[:current_user], :read_issue, issue.moved_to)
+      end
+
       expose(:has_tasks) do |issue, _|
         !issue.task_list_items.empty?
       end