In Sortable, save the z-index before opacity to make sure we get the right value

[CyborgMaster]

We need to make sure to grab the zIndex before setting the opacity, because setting the opacity to anything lower than 1 causes the zIndex to change from "auto" to 0. The later when we put it back, we think the original value was 0 and this line doesn't work properly.

this.helper.css( "zIndex", this._storedZIndex === "auto" ? "" : this._storedZIndex );

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