Add ActivityPub actor representing the entire server (#11321)

* Add support for an instance actor

* Skip username validation for local Application accounts

* Add migration script to create instance actor

* Make Codeclimate happy

* Switch to id -99 for instance actor

* Remove unused `icon` and `image` attributes from instance actor

* Use if/elsif/else instead of return + ternary operator

* Add instance actor to fresh installs

* Use instance actor as instance representative

Use instance actor for forwarding reports, relay operations, and spam
auto-reporting.

* Seed database in test environment

* Fix single-user mode

* Fix tests

* Fix specs to accomodate for an extra `Account`

* Auto-reject follows on instance actor

Following an instance actor might make sense, but we are not handling that
right now, so auto-reject.

* Fix webfinger lookup and serialization for instance actor

* Rename instance actor

* Make it clear in the HTML view that the instance actor should not be blocked

* Raise cache time for instance actor as there's no dynamic content

* Re-use /about/more with a flash message for instance actor profile

app/controllers/about_controller.rb

@@ -11,7 +11,9 @@ class AboutController < ApplicationController
 
   def show; end
 
-  def more; end
+  def more
+    flash.now[:notice] = I18n.t('about.instance_actor_flash') if params[:instance_actor]
+  end
 
   def terms; end
 

app/controllers/application_controller.rb

@@ -91,7 +91,7 @@ class ApplicationController < ActionController::Base
   end
 
   def single_user_mode?
-    @single_user_mode ||= Rails.configuration.x.single_user_mode && Account.exists?
+    @single_user_mode ||= Rails.configuration.x.single_user_mode && Account.where('id > 0').exists?
   end
 
   def use_seamless_external_login?

app/controllers/home_controller.rb

@@ -58,7 +58,7 @@ class HomeController < ApplicationController
     if request.path.start_with?('/web')
       new_user_session_path
     elsif single_user_mode?
-      short_account_path(Account.local.without_suspended.first)
+      short_account_path(Account.local.without_suspended.where('id > 0').first)
     else
       about_path
     end

app/controllers/instance_actors_controller.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class InstanceActorsController < ApplicationController
+  include AccountControllerConcern
+
+  def show
+    expires_in 10.minutes, public: true
+    render json: @account, content_type: 'application/activity+json', serializer: ActivityPub::ActorSerializer, adapter: ActivityPub::Adapter, fields: restrict_fields_to
+  end
+
+  private
+
+  def set_account
+    @account = Account.find(-99)
+  end
+
+  def restrict_fields_to
+    %i(id type preferred_username inbox public_key endpoints url manually_approves_followers)
+  end
+end