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

spec/services/fetch_resource_service_spec.rb

@@ -1,8 +1,6 @@
 require 'rails_helper'
 
 RSpec.describe FetchResourceService, type: :service do
-  let!(:representative) { Fabricate(:account) }
-
   describe '#call' do
     let(:url) { 'http://example.com' }
 
@@ -60,7 +58,7 @@ RSpec.describe FetchResourceService, type: :service do
 
       it 'signs request' do
         subject
-        expect(a_request(:get, url).with(headers: { 'Signature' => /keyId="#{Regexp.escape(ActivityPub::TagManager.instance.uri_for(representative) + '#main-key')}"/ })).to have_been_made
+        expect(a_request(:get, url).with(headers: { 'Signature' => /keyId="#{Regexp.escape(ActivityPub::TagManager.instance.uri_for(Account.representative) + '#main-key')}"/ })).to have_been_made
       end
 
       context 'when content type is application/atom+xml' do