create a card/task clone/duplicate from one board to another

[arscynic]

I have a personal Kanban board and one shared with a colleague. Currently, I click "…" and use "Move card" to the shared board if a task is relevant for the both of us. However, the fact that this certain card is now not present anymore in my personal board requires me to switch between the two more frequently than needed.

It would be an extremely convenient feature to have card/task clones where change in one card changes the other.

Another solution might be that cards with a user assigned to them can get cloned to one's personal board.

[jringoot]

(Without looking into the code (sorry :-)))
If boards are populated by the references to cards,
then the references can be reused on other boards.

[juliusknorr]

Implemented with #6452