This commit moves the rendering of the button back to the server, and
shows/hides it using opacity rather than display. It also removes the
transform applied to the button on hover (scale). Previously, both of these
factors automatically triggered a reflow, which creates a performance
bottleneck on pages with larger DOM size.
MR: !12103
Bryce Johnson
Eric Eastwood
Alfredo Sumaran
Mike Greiling
Phil Hughes