* 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
