Replace "Read" / "Mark read" with a state that is unambiguous

On source pages a black "Read" and a white "Mark read" left the reader unsure which state they were in. One control: "Mark as read" (unread state) that becomes "Read ✓ · mark unread" (read state), and the page's byline shows the same state in words.

Done when: no page shows two buttons for one state.