Respecting the --nosearchbar option of kiwix-serve

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.

static/templates/viewer_settings.js

@@ -1,4 +1,5 @@
 const viewerSettings = {
+  toolbarEnabled:       {{enable_toolbar}},
   linkBlockingEnabled:  {{enable_link_blocking}},
   libraryButtonEnabled: {{enable_library_button}}
 }