When users hover over a column trigger menu, show a "preview" with the rules instead of a tooltip

Summary:
Ref T5474. The first rough cut of triggers showed some of the trigger rules in a tooltip when you hover over the "add/remove" trigger menu.

This isn't great since we don't have much room and it's a bit finnicky / hard to read.

Since we have a better way to show effects now in the drop preview, just use that instead. When you hover over the trigger menu, preview the trigger in the "drop effect" element, with a "Trigger: such-and-such" header.

Test Plan:

• This is pretty tough to screenshot.
• Hovered over menu, got a sensible preview of the trigger effects.
• Dragged a card over the menu, no preview.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T5474

Differential Revision: https://secure.phabricator.com/D20304