Expose archive status of projects in API

That way clients like Gitlab CI can decide to show or hide projects
based on that information

lib/api/entities.rb

@@ -43,6 +43,7 @@ module API
     class Project < Grape::Entity
       expose :id, :description, :default_branch
       expose :public?, as: :public
+      expose :archived?, as: :archived
       expose :visibility_level, :ssh_url_to_repo, :http_url_to_repo, :web_url
       expose :owner, using: Entities::UserBasic, unless: ->(project, options) { project.group }
       expose :name, :name_with_namespace