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/lib/webfinger_resource.rb

@@ -23,11 +23,17 @@ class WebfingerResource
   def username_from_url
     if account_show_page?
       path_params[:username]
+    elsif instance_actor_page?
+      Rails.configuration.x.local_domain
     else
       raise ActiveRecord::RecordNotFound
     end
   end
 
+  def instance_actor_page?
+    path_params[:controller] == 'instance_actors'
+  end
+
   def account_show_page?
     path_params[:controller] == 'accounts' && path_params[:action] == 'show'
   end