revoke shares instead of deleting

[dartcafe]

Changed the deletion of shares

• shares without votes get deleted imediately (without the timer)
• shares with votes can be revoked, so that the sharee is still able to view the results, but any other interaction is denied.
• revoked shares can be reactivated
• admin rights are also withdrawn, if a share with admin rights for the poll gets revoked
• deleting revoked shares will also remove the sharees votes
• refactored the acl to respect the changes (further optimizations in another pr)

todo:

• delete votes when deleting revoked shares