Don't show workboard action previews if the action won't have any effect

Summary:
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.

Test Plan:
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.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T10335

Differential Revision: https://secure.phabricator.com/D20300