Auto-collapse empty kanban stacks

[Aaron]

Title is self explanatory. Would save space and scrolling time for boards with many stacks.

[Emar_Vegt1]

This seems to be the default now, which presents another pretty fundamental problem: with empty stacks hidden, there is no way to move a card to one of those stacks! Meaning a card can only be moved to a stack that already has at least one card in it.

Essentially, that makes the Kanban view not a Kanban view anymore. 

[codemutation]

This should definitely be an option that can be toggled either way.