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

db/migrate/20190715164535_add_instance_actor.rb

@@ -0,0 +1,9 @@
+class AddInstanceActor < ActiveRecord::Migration[5.2]
+  def up
+    Account.create!(id: -99, actor_type: 'Application', locked: true, username: Rails.configuration.x.local_domain)
+  end
+
+  def down
+    Account.find_by(id: -99, actor_type: 'Application').destroy!
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_07_06_233204) do
+ActiveRecord::Schema.define(version: 2019_07_15_164535) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"

db/seeds.rb

@@ -1,7 +1,9 @@
 Doorkeeper::Application.create!(name: 'Web', superapp: true, redirect_uri: Doorkeeper.configuration.native_redirect_uri, scopes: 'read write follow')
 
+domain = ENV['LOCAL_DOMAIN'] || Rails.configuration.x.local_domain
+Account.create!(id: -99, actor_type: 'Application', locked: true, username: domain)
+
 if Rails.env.development?
-  domain = ENV['LOCAL_DOMAIN'] || Rails.configuration.x.local_domain
   admin  = Account.where(username: 'admin').first_or_initialize(username: 'admin')
   admin.save(validate: false)
   User.where(email: "admin@#{domain}").first_or_initialize(email: "admin@#{domain}", password: 'mastodonadmin', password_confirmation: 'mastodonadmin', confirmed_at: Time.now.utc, admin: true, account: admin, agreement: true, approved: true).save!