Dim the action drop preview element when the cursor approaches

Summary:
Depends on D20308. Ref T5474. The element which previews what will happen when you drop a task somewhere can cover the bottom part of the rightmost column on a workboard.

To fix this, I'm trying to just fade it out if you put your cursor over it. I tried to do this in a simple way previously (":hover" + "opacity: 0.25") but it doesn't actually work because "pointer-events: none" stops ":hover" from working.

Instead, do this in Javascript. This is a little more complicated but: it works; and we can do the fade when you get near the element instead of actually over it, which feels a little better.

Test Plan:

• Shrank window to fairly small size so that the preview could cover up stuff on the workboard.
• Dragged a card toward the rightmost column.
• Before: drop action preview covered some workboard stuff.
• After: preview faded out as my cursor approached.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T5474

Differential Revision: https://secure.phabricator.com/D20320