If `kiwix-serve` is run with the `--nosearchbar` option the toolbar is
disabled (hidden) in its viewer.
Note however that certain actions performed by the viewer merely with
the purpose of keeping the toolbar up-to-date are still carried out.
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`.