Add draft locale support to Help Center portals so unpublished locales are hidden from the public portal

Add draft locale support to Help Center portals so unpublished locales are hidden from the public portal: a task in bird-critic-1.5-full-300 (Harbor dataset). Teams preparing localized Help Center content should be able to work on a locale in the admin dashboard without that locale appearing in…

The task

Teams preparing localized Help Center content should be able to work on a locale in the admin dashboard without that locale appearing in the public portal's language switcher. This PR adds `draft_locales` as a subset of `allowed_locales` in the portal config and enforces that the `default_locale` cannot be drafted.

Part of kochsnow/bird-critic-1.5-full-300.