Add an "Sort by Creation Date" filter to workboards and modularize remaining order behaviors
Allow columns to be sorted by creation date, so the newest tasks rise to the top.
In this ordering you can never reposition cards, since editing a creation date by dragging makes no sense. This will be true of the "points" ordering too (although we could imagine doing something like prompting the user, some day).
Test Plan: Viewed boards by "natural" (allows reordering both when dragging within and between columns), "priority" (reorder only within columns), and "creation date" (reorder never). Dragged cards around between and within columns, got apparently sensible behavior.
Reviewed By: amckinley
Maniphest Tasks: T10578
Differential Revision: https://secure.phabricator.com/D20275