Reference and search improvements

[tw4l]

• [x] Load spine item prior to going to reference to ensure it always works, even if chapter hasn't been visited in this session yet
• [x] Highlight paragraph corresponding to reference (implemented: highlights first character of paragraph as indicator; might be able to come back to highlighting the whole thing later but it proved to be difficult/a big time sink so decided to move on for now)
• [X] Override browser's ctrl/cmd-f behavior; display in-app search sidebar and focus on input instead of opening browser find widget