R1:6138e50962e6

When moving cards on workboards, treat before/after cards as optional hints, not strict requirements

Summary:
Depends on D20320. Ref T12175. Ref T13074. Currently, when you move a card between columns, the internal transaction takes exactly one `afterPHID` or `beforePHID` and moves the card before or after the specified card.

This is a fairly strict interpretation and causes a number of practical issues, mostly because the user/client view of the board may be out of date and the card they're dragging…