Make it possible to suppress warning when you navigate away

[jan-niestadt]

Provide an option to not warn the user when they are about to navigate away from the application.

Unfortunately, this can't be a checkbox below the warning itself, due to the way browsers handle this - it will have to be a separate option somewhere. Maybe there could be an Options button for this.

(maybe the "search case- and accent-sensitively" option could be moved to that dialog as well..? Having a checkbox with text is almost certainly clearer than a button labelled "Aaa")