Allow to dereference Follow object for ActivityPub (#5772)

* Allow to dereference Follow object for ActivityPub

* Accept IRI as object representation for Accept activity

app/controllers/activitypub/follows_controller.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class ActivityPub::FollowsController < Api::BaseController
+  include SignatureVerification
+
+  def show
+    render(
+      json: FollowRequest.includes(:account).references(:account).find_by!(
+        id: params.require(:id),
+        accounts: { domain: nil, username: params.require(:account_username) },
+        target_account: signed_request_account
+      ),
+      serializer: ActivityPub::FollowSerializer,
+      adapter: ActivityPub::Adapter,
+      content_type: 'application/activity+json'
+    )
+  end
+end