d2l-sticky-element's z-index is too high

[bemailloux]

It has a z-index of 4: https://github.com/BrightspaceUI/table/blob/master/d2l-sticky-element.js#L23

This is causing the button to appear on top of other elements such as the immersive navbar, which only has a z-index of 2:

As far as I can tell a z-index of 1 for the sticky element might be sufficient.