Fix a few issues in ToC
- Update the currently-highlighted ToC section when *starting* to pull out the drawer, instead of after it's already pulled out.
- On the JS side, fix the detection of positioning in the Lead section.
- Fix highlighting of the correct section in the ToC when the article has subsections that are "deeper" than H3. Since we hide these subsections in the ToC, and we were previously using the sectionID returned by the WebView as an index into the ToC list, it was causing the incorrect section to be highlighted.