Add numbers to cards

[PhMemmel]

For referencing cards within one kanban activity it would be nice if the cards had numbers.

Additional thoughts:

• Add different (configurable) prefixes for different boards within an activity, so there would be the card PUB-1, PUB-25 in the public board, XYZ-1, XYZ-2, ... in one personal board, ABC-1, ABC-2, ABC-3, ... in a group board etc.
  Not sure if this is a good idea, maybe a common prefix for all the boards within an activity would the better solution.
• If cards are referenced within the title or description of other cards, this will be converted to a link to the description of the other card
• An additional possibility of linking cards together would be ideal