Add server banner to web app, add GET /api/v2/instance to REST API (#19294)

app/controllers/api/v2/instances_controller.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+class Api::V2::InstancesController < Api::V1::InstancesController
+  def show
+    expires_in 3.minutes, public: true
+    render_with_cache json: InstancePresenter.new, serializer: REST::InstanceSerializer, root: 'instance'
+  end
+end