Profile redirect notes (#5746)

* Serialize moved accounts into REST and ActivityPub APIs

* Parse federated moved accounts from ActivityPub

* Add note about moved accounts to public profiles

* Add moved account message to web UI

* Fix code style issues

db/migrate/20171118012443_add_moved_to_account_id_to_accounts.rb

@@ -0,0 +1,6 @@
+class AddMovedToAccountIdToAccounts < ActiveRecord::Migration[5.1]
+  def change
+    add_column :accounts, :moved_to_account_id, :bigint, null: true, default: nil
+    add_foreign_key :accounts, :accounts, column: :moved_to_account_id, on_delete: :nullify
+  end
+end