v3.1.0

• [#736] Existing valid tokens are now reused in client_credentials flow
• [#749] Allow user to raise authorization error with custom messages.
  Under resource_owner_authenticator block a user can
  raise Doorkeeper::Errors::DoorkeeperError.new('custom_message')
• [#762] Check doesn’t abort the actual migration, so it runs
• [#722] doorkeeper_forbidden_render_options now supports returning a 404 by
  specifying respond_not_found_when_forbidden: true in the
  doorkeeper_forbidden_render_options method.
• [#734] Simplify and remove duplication in request strategy classes