Allow the author of a card to initially hide it from other users

[bastf]

Hi, while testing Scrumlr I realised that it would be extremely useful to have the option to hide your own cards from other users initially (regardless of the role). This would enable different types of retrospectives.

Currently, board owners can only show or hide all cards from other authors globally for all participants. With this extension, participants could decide when to reveal cards to others. This would enable a meeting style in which not all cards are revealed at once, but rather one by one. Once a card has been revealed, it cannot be hidden again.

I'm not sure if this is compatible with the way how Scrumlr is supposed to work. Thanks a lot for considering my suggestion!