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

config/routes.rb

@@ -28,6 +28,10 @@ Rails.application.routes.draw do
   get 'intent', to: 'intents#show'
   get 'custom.css', to: 'custom_css#show', as: :custom_css
 
+  resource :instance_actor, path: 'actor', only: [:show] do
+    resource :inbox, only: [:create], module: :activitypub
+  end
+
   devise_scope :user do
     get '/invite/:invite_code', to: 'auth/registrations#new', as: :public_invite
     match '/auth/finish_signup' => 'auth/confirmations#finish_signup', via: [:get, :patch], as: :finish_signup