Make waving cover files around on boards more reliable

Summary:
Currently, in Safari, if you drag an image onto a board to make it a cover file and then wave it around wildly a lot over differnent cards, it sometimes glitches out a bit and won't drop on them properly.

This appears to be because sequencing and delivery of dragenter/dragleave events isn't always totally ideal.

Instead, just cancel any existing drag when we get a new drag that targets a new drop target.

Test Plan:

• Opened a board with a bunch of cards.
• Dragged a file from my desktop onto the board.
• Waved it around wildly, hovering over many different cards.
  • Before patch: sometimes cards under the cursor stopped highlighting properly.
  • After patch: behavior seems correct and consistent.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15208