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

app/services/activitypub/fetch_featured_collection_service.rb

@@ -0,0 +1,52 @@
+# frozen_string_literal: true
+
+class ActivityPub::FetchFeaturedCollectionService < BaseService
+  include JsonLdHelper
+
+  def call(account)
+    @account = account
+    @json    = fetch_resource(@account.featured_collection_url, true)
+
+    return unless supported_context?
+    return if @account.suspended? || @account.local?
+
+    case @json['type']
+    when 'Collection', 'CollectionPage'
+      process_items @json['items']
+    when 'OrderedCollection', 'OrderedCollectionPage'
+      process_items @json['orderedItems']
+    end
+  end
+
+  private
+
+  def process_items(items)
+    status_ids = items.map { |item| value_or_id(item) }
+                      .reject { |uri| ActivityPub::TagManager.instance.local_uri?(uri) }
+                      .map { |uri| ActivityPub::FetchRemoteStatusService.new.call(uri) }
+                      .compact
+                      .select { |status| status.account_id == @account.id }
+                      .map(&:id)
+
+    to_remove = []
+    to_add    = status_ids
+
+    StatusPin.where(account: @account).pluck(:status_id).each do |status_id|
+      if status_ids.include?(status_id)
+        to_add.delete(status_id)
+      else
+        to_remove << status_id
+      end
+    end
+
+    StatusPin.where(account: @account, status_id: to_remove).delete_all unless to_remove.empty?
+
+    to_add.each do |status_id|
+      StatusPin.create!(account: @account, status_id: status_id)
+    end
+  end
+
+  def supported_context?
+    super(@json)
+  end
+end