Bug 1069992 - [E-Mail] Wobbly Refresh Icon Animation

Give the button element a fixed size that matches the background size, and instead of position absolute approach, rely on the auto centering that was already possible within the container and by `margin: 0 auto`.

Confirmed that the icon is in the same place as before by comparing a before and after change screenshot overlaid on top of each other. The icon is no longer wobbly. I believe it was hard for the rotation transform to calculate precise position with the width: 100%. Once I fixed the width/height to exact measurements it cleared up the wobble.