Add button to re-subscribe to all accounts from a domain (#4285)

app/controllers/admin/instances_controller.rb

@@ -6,15 +6,26 @@ module Admin
       @instances = ordered_instances
     end
 
+    def resubscribe
+      params.require(:by_domain)
+      Pubsubhubbub::SubscribeWorker.push_bulk(subscribeable_accounts.pluck(:id))
+      redirect_to admin_instances_path
+    end
+
     private
 
     def paginated_instances
       Account.remote.by_domain_accounts.page(params[:page])
     end
+
     helper_method :paginated_instances
 
     def ordered_instances
       paginated_instances.map { |account| Instance.new(account) }
     end
+
+    def subscribeable_accounts
+      Account.with_followers.remote.where(domain: params[:by_domain])
+    end
   end
 end