R1:75a5dd8d8cc0

Add more accessibility labels for screen readers

Summary:
Depends on D19594. See PHI823. Ref T13164.

- Add a label for the "X" button in comment areas, like "Remove Action: Change Subscribers".
- Add a label for the floating header display options menu in Differential.
- Add `role="button"` to `PHUIButtonView` objects that we render with an `<a ...>` tag.

Test Plan:
Viewed a revision with `?__aural__=true`:

- Saw "Remove Action: ..." label.
- Saw "Display Options" label.
- Used inspector to…