fixes https://github.com/TryGhost/Team/issues/2779 fixes https://github.com/TryGhost/Team/issues/2781 Needs some more manual testing, unit tests and E2E tests. But the exporting is implemented and some columns are removed based on the site settings.
fixes https://github.com/TryGhost/Team/issues/2780 refs https://github.com/TryGhost/Team/issues/2781 Adds an export button to the posts page in admin (behind feature flag). It downloads a placeholder CSV via a real endpoint (`/posts/export`).
fixes https://github.com/TryGhost/Team/issues/2778 It is easier to add extra classes using the latest patterns if it has its own package.