Date ambiguity sliders get stuck when the left slider is dragged all the way to the right

[davidmcclure]

The problem the right slider is statically set to have a higher z-index, so when the left slider is pulled all the way to the left is gets slipped beneath the right slider. But then the right slider can't be pulled righwards "off" of the slider track, so neither slider can be moved at all.

To fix, on mousedown on either of the sliders set that slider's z-index to 2 (or whatever) and the other slider to 1 (or whatever). Do that toggle on each mousedown, and the most recently dragger slider will always be accessible to the cursor, meaning that any action can always be undone.

[davidmcclure]

Fixed by 6f013c2.