- Align copy with normal markdown editing experience: The button to
  enter full screen says "Go full screen." The button to exit full
  screen has been changed from "Leave zen mode" to "Exit full screen".
  Zen mode is not a term that is presented to the user otherwise.
- Align button style with normal markdown experience. The button should
  be styled like an icon button.
- Use a tooltip. The button to enter full screen mode uses a tooltip,
  and this button should use one as well -- especially since this button
  is the only clickable affordance in the entire UI that the user can
  use to escape this focus mode.
- Fix alignment. The button was made too tall because it inherited a
  line height from a parent.
- Increase size. The button was too small compared to the larger text
  editor in full screen mode.

Changelog: fixed