[mod] new preference: query_in_title

* disable by default
* settings.yml: ui.query_in_title
* in /preferences: privacy tab

when enabled, the result page's title contains the user query.

previously:
* oscar theme: the query was always included
* simple theme: the query was included with the GET method

searx/settings_defaults.py

@@ -186,6 +186,7 @@ SCHEMA = {
         },
         'results_on_new_tab': SettingsValue(bool, False),
         'advanced_search': SettingsValue(bool, False),
+        'query_in_title': SettingsValue(bool, False),
         'categories_order': SettingsValue(list, CATEGORY_ORDER),
     },
     'preferences': {