Add external_content to list of formats without rendering apps
Add external_content to list of formats without rendering apps

The `external_content` schema and document type represent a non-GOV.UK
URL which will be used for things like search indexing. Items of this
type have a URL but no rendering_app.

See [RFC 88](https://github.com/alphagov/govuk-rfcs/pull/88) for more
details.

alphagov/publishing-api,alphagov/publishing-api