Made the viewer respect the `--blockexternal` and `--nolibrarybutton`
options of `kiwix-serve`. Those options are passed to the viewer
via the dynamically generated resource `/viewer_settings.js`.
Before this fix there were two issues with the taskbar search box:
1. The book used for the suggestions API was resolved only once during
the page load and didn't change during navigation.
2. The current book could not be resolved from a search URL.
Now both issues are fixed.