Add locality check to ActivityPub::FetchRemoteAccountService (#9109)

* Add locality check to ActivityPub::FetchRemoteAccountService

Fix #8643

Because there are a few places where it is called, it is difficult
to confirm if they all previously checked it for locality. It's better
to make sure within the service.

* Remove faux-remote duplicates of local accounts

db/migrate/20181026034033_remove_faux_remote_account_duplicates.rb

@@ -0,0 +1,16 @@
+class RemoveFauxRemoteAccountDuplicates < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    local_domain = Rails.configuration.x.local_domain
+
+    # Just a safety measure to ensure that under no circumstance
+    # we will query `domain IS NULL` because that would return
+    # actually local accounts, the originals
+    return if local_domain.nil?
+
+    Account.where(domain: local_domain).in_batches.destroy_all
+  end
+
+  def down; end
+end