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strict interpretation and causes a number of practical issues, mostly because the user/client view of the board may be out of date and the card they're dragging before or after may no longer exist: another user might have moved or hidden it between the last client update and the current time.

In T13074, we also run into a more subtle issue where a card that incorrectly appears in multiple columns fatals when dropped before or after itself.

In all cases, a better behavior is just to complete the move and accept that the position may not end up exactly like the user specified. We could prompt the user instead:

You tried to drop this card after card X, but that card has moved since you last loaded the board. Reload the board and try again.

...but this is pretty hostile and probably rarely/never what the user wants.

Instead, accept a list of before/after PHIDs and just try them until we find one that works, or accept a default position if none work. In essentially all cases, this means that the move "just works" like users expect it to instead of fataling in a confusing/disruptive/undesirable (but "technically correct") way.

(A followup will make the client JS send more beforePHIDs/afterPHIDs so this works more often.)

We could eventually add a "strict" mode in the API or something if there's some bot/API use case for precise behavior here, but I suspect none exist today or are (ever?) likely to exist in the future.

Test Plan:

• (T13074) Inserted two conflicting rows to put a card on two columns on the same board. Dropped one version of it underneath the other version. Before: confusing fatal. After: cards merge sensibly into one consistent card.
• (T12175) Opened two views of a board. Moved card A to a different column on the first view. On the second view, dropped card B under card A (still showing in the old column). Before: confusing fatal. After: card ended up in the right column in approximately the right place, very reasonably.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13074, T12175

Differential Revision: https://secure.phabricator.com/D20321