12. Use publish workflow to not show not yet confirmed speakers

[dbu]

• Have your content implement PublishableInterface and/or PublishTimePeriodInterface (already the case for the base Page document)
• Check cmf_is_published() before displaying content
  • Direct URLs are checked automatically (open a presentation, unpublish it, hit reload and you get a 404)
• Enable admin extension for controlling publish information (rename publishable on speaker to "Confirmed")
• Custom publish voter PresentationVoter to not show presentations if no speaker is published (confirmed) yet.