Don't show workboard action previews if the action won't have any effect
Ref T10335. When you (for example) drag a "Resolved" task into a column with "Trigger: change status to resolved.", don't show a hint that the action will "Change status to resolved." since this isn't helpful and is somewhat confusing.
For now, the only visibility operator is "!=" since all current actions are simple field comparisons, but some actions in the future (like "add subscriber" or "remove project") might need other conditions.
Dragged cards in ways that previously provided useless hints: move from column A to column B on a "Group by Priority" board; drag a resolved task to a "Trigger: change status to as resolved" column. Saw a more accurate preview in both cases.
Drags which actually cause effects still show the effects correctly.
Reviewed By: amckinley
Maniphest Tasks: T10335
Differential Revision: https://secure.phabricator.com/D20300