Add new SHOW_WEB_SUGGESTIONS system setting, referenced by the search settings

(in a follow-on change). This setting, to be respected by global search, and which
I will also file a bug to the browser to respect, determines whether live web
suggestions will be shown to the user as they type. For privacy reasons, this
could be considered undesirable, as partial queries can then be sent to the user's
chosen search engine, so a setting was required.