The compose expand / collapse button is moved from the top right of the
compose area to the top right corner inside the compose textarea /
preview.
The textarea / preview and the button area shares the same grid parent.
25px extra padding is added to and margin is subtracted from the right
of the element so the button is visually inside it. It appears only on
hovering the compose area, and uses a new custom icon.
Fixes: #28791.
N-Shar-ma