Create instance actor if it hasn't been properly seeded (mastodon#15693)

An uncommon but somewhat difficult to digagnose issue is dealing with
improperly-seeded databases. In such cases, instance-signed fetches will
fail with a ActiveRecord::RecordNotFound error, usually caught and handled
as generic 404, leading people to think the remote resource itself has not
been found, while it's the local instance actor that does not exist.

This commit changes the code so that failure to find the instance actor
automatically creates a new one, so that improperly-seeded databases do
not cause any issue.

app/controllers/instance_actors_controller.rb

@@ -13,7 +13,7 @@ def show
  private
 
  def set_account
- @account = Account.find(-99)
+ @account = Account.representative
  end
 
  def restrict_fields_to

app/models/concerns/account_finder_concern.rb

@@ -14,6 +14,8 @@ def find_remote!(username, domain)
 
  def representative
  Account.find(-99)
+ rescue ActiveRecord::RecordNotFound
+ Account.create!(id: -99, actor_type: 'Application', locked: true, username: Rails.configuration.x.local_domain)
  end
 
  def find_local(username)