Federate pinned statuses over ActivityPub (#6610)

* Federate pinned statuses over ActivityPub

* Display pinned toots in web UI

Fix #6117

* Fix migration

* Fix tests

* Update outbox_serializer.rb

* Update remove_serializer.rb

* Update add_serializer.rb

* Update fetch_featured_collection_service.rb

config/routes.rb

@@ -58,8 +58,10 @@ Rails.application.routes.draw do
     resources :following, only: [:index], controller: :following_accounts
     resource :follow, only: [:create], controller: :account_follow
     resource :unfollow, only: [:create], controller: :account_unfollow
+
     resource :outbox, only: [:show], module: :activitypub
     resource :inbox, only: [:create], module: :activitypub
+    resources :collections, only: [:show], module: :activitypub
   end
 
   resource :inbox, only: [:create], module: :activitypub