Delete unused Block fields

[BeritJanssen]

After #973

• block.title no longer required but optional (is not necessary per se, except for when you use dashboard)
• remove block.consent field (is on experiment level now)
• remove block.active field (we can use the active field on an experiment level to deactivate the whole experiment)
  - remove block.slug field

[drikusroor]

remove block.slug field

Will slug not be used anymore at all?

[BeritJanssen]

I thought we might use id instead, to avoid confusion with handling slugs at different levels.