Support per-column sorting

[Alanaktion]

Currently the entire Kanban board keeps cards in the sort order from the backlog, which can limit its usability with many workflows. The ideal way to solve this is probably to update the upstream backlog sorting system to allow arbitrary sort changes to single items, e.g. storing each item as a float value relative to the ones near it. Alternatively, this could be implemented specifically in the Kanban board itself and have it store a separate per-column sort order not associated with the main backlog sorting.