Homestyx hydra

R1:4dd6a1224d36

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…
Repository: R1 hydra
Commit Date: Feb 8 2016