This makes a few changes to the help popup:
- Move css to chrome.css, since this is a UI element.
- Move HTML code to index.hbs instead of generated in JavaScript.
In general I prefer to keep HTML out of JavaScript when possible,
and I didn't see a particular reason to avoid it.
- Added a click handler to dismiss the popup.
- Make sure handlers get removed when dismissed.
- Use `mdbook-` prefixes for IDs to avoid collisions with headers.
- Don't show search if it isn't enabled.
- Add the new `/` shortcut.
- Use flex layout for better positioning.
- Dim out the surrounding text using an overlay.
- Various other styling tweaks.
- Add a GUI test.
