WC Post Types, Blocks: Add support for private speakers & sessions

[ryelle]

Fixes #717 — This is a simpler approach to the issue described, where it's difficult to style the speakers & sessions without making them public. By enabling private sessions & speakers to be used in blocks, this lets organizers (who have permissions to see private content) view and edit the content, without showing sessions & speakers to site visitors, and without notifying the wporg profiles code.

The intention to only show private sessions and not all sessions is that all sessions are submitted as drafts, so this lets the organizer team update and privately publish the accepted talks, while leaving the rest alone.

Screenshots

Private speakers now show up in the sessions list table, with updated label.

Private sessions appear in the Speaker Sessions block, which can be used on speaker posts.

And the main use for this, private sessions will show up in the schedule. In the following screenshot, the "Friday keynote" is a privately published regular session (the rest are public custom sessions).

How to test the changes in this Pull Request:

1. Add some sessions and speakers
2. Publish them privately
3. The should be visible on the frontend when logged in, but not when logged out