Move drag ghosts above dialogs

Summary: These currently have a z-index which puts them beneath dialogs, which doesn't work well for dialogs like "Reorder Columns" on workboards.

Test Plan:

• Dragged columns on a workboard in the "Reorder Columns" dialog.
• Dragged normal stuff, too.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15197