Improved remote thread fetching (#10106)

* Fetch up to 5 replies when discovering a new remote status

This is used for resolving threads downwards. The originating
server must add a “replies” attributes with such replies for it to
be useful.

* Add some tests for ActivityPub::FetchRepliesWorker

* Add specs for ActivityPub::FetchRepliesService

* Serialize up to 5 public self-replies for ActivityPub notes

* Add specs for ActivityPub::NoteSerializer

* Move exponential backoff logic to a worker concern

* Fetch first page of paginated collections when fetching thread replies

* Add specs for paginated collections in replies

* Move Note replies serialization to a first CollectionPage

The collection isn't actually paginable yet as it has no id nor
a `next` field. This may come in another PR.

* Use pluck(:uri) instead of map(&:uri) to improve performances

* Fix fetching replies when they are in a CollectionPage

app/serializers/activitypub/note_serializer.rb

@@ -13,6 +13,8 @@ class ActivityPub::NoteSerializer < ActiveModel::Serializer
   has_many :media_attachments, key: :attachment
   has_many :virtual_tags, key: :tag
 
+  has_one :replies, serializer: ActivityPub::CollectionSerializer
+
   def id
     ActivityPub::TagManager.instance.uri_for(object)
   end
@@ -33,6 +35,17 @@ class ActivityPub::NoteSerializer < ActiveModel::Serializer
     { object.language => Formatter.instance.format(object) }
   end
 
+  def replies
+    ActivityPub::CollectionPresenter.new(
+      type: :unordered,
+      first: ActivityPub::CollectionPresenter.new(
+        type: :unordered,
+        page: true,
+        items: object.self_replies(5).pluck(:uri)
+      )
+    )
+  end
+
   def language?
     object.language.present?
   end