Add outbox attribute to instance actor (mastodon#14721)

It's not useful for now, but it's required by ActivityPub

app/controllers/activitypub/outboxes_controller.rb

@@ -20,9 +20,9 @@ def show
   def outbox_presenter
     if page_requested?
       ActivityPub::CollectionPresenter.new(
-        id: account_outbox_url(@account, page_params),
+        id: outbox_url(page_params),
         type: :ordered,
-        part_of: account_outbox_url(@account),
+        part_of: outbox_url,
         prev: prev_page,
         next: next_page,
         items: @statuses
@@ -32,12 +32,20 @@ def outbox_presenter
         id: account_outbox_url(@account),
         type: :ordered,
         size: @account.statuses_count,
-        first: account_outbox_url(@account, page: true),
-        last: account_outbox_url(@account, page: true, min_id: 0)
+        first: outbox_url(page: true),
+        last: outbox_url(page: true, min_id: 0)
       )
     end
   end
 
+  def outbox_url(**kwargs)
+    if params[:account_username].present?
+      account_outbox_url(@account, **kwargs)
+    else
+      instance_actor_outbox_url(**kwargs)
+    end
+  end
+
   def next_page
     account_outbox_url(@account, page: true, max_id: @statuses.last.id) if @statuses.size == LIMIT
   end
@@ -65,4 +73,8 @@ def page_requested?
   def page_params
     { page: true, max_id: params[:max_id], min_id: params[:min_id] }.compact
   end
+
+  def set_account
+    @account = params[:account_username].present? ? Account.find_local!(username_param) : Account.representative
+  end
 end

app/controllers/instance_actors_controller.rb

@@ -17,6 +17,6 @@ def set_account
   end
 
   def restrict_fields_to
-    %i(id type preferred_username inbox public_key endpoints url manually_approves_followers)
+    %i(id type preferred_username inbox outbox public_key endpoints url manually_approves_followers)
   end
 end

app/serializers/activitypub/actor_serializer.rb

@@ -74,7 +74,7 @@ def devices
   end
 
   def outbox
-    account_outbox_url(object)
+    object.instance_actor? ? instance_actor_outbox_url : account_outbox_url(object)
   end
 
   def featured

config/routes.rb

@@ -37,6 +37,7 @@
 
   resource :instance_actor, path: 'actor', only: [:show] do
     resource :inbox, only: [:create], module: :activitypub
+    resource :outbox, only: [:show], module: :activitypub
   end
 
   devise_scope :user do